College graduates are now leaving school with not only a diploma, but also with more debt than in any other period in history. According to the College Board, average financial aid per full time equivalent student adjusted for inflation, has almost doubled in the last decade. Additionally, not only is the average student debt load higher, but also the number of loans originated increased by nearly fourfold. Thus student lending has increased in breadth as well as depth, doubly impacting the financial situations of those students who must depend on student loans to finance their education. Furthermore, these figures no doubt understate the true level of indebtedness incurred by students and their families since some undoubtedly finance part of their education and living expenses through the use of consumer loans, home equity loans or credit cards. Thus, the already huge burden of student loan debt incurred by students and their families is exacerbated by financial obligations from other sources. This situation is the outcome of a precarious combination of easy credit, thanks to card issuers who are eager to tap into this lucrative market, financial naiveté on the part of students, and a sense on both sides that, if worse comes to worse, mommy and daddy will step in and set things right. All combine to make credit a tempting lure to students strapped for cash. While credit still remain a virtual element for the financing of many students’ education at expensive colleges, its implications must carefully weighed and considered, lest the cash-poor student find himself with more that he bargained for. Most students in the United States attend colleges whose tuition does not exceed $15,000, however. Nevertheless, rising prices, combined with stagnant income gains among low-and moderate-income families, have made it difficult if not impossible for a student from a family of such an income level, a student who has the most to gain from a college education, to attend college without significantly adding to the financial burden of his or her family. While student loans still remain a vital element for the financing of many students’ education at expensive colleges, its implications must be carefully weighed and considered, lest the cash-poor student find himself with more than he bargained for.In Paragraph 1, "... the number of loans originated increased by nearly fourfold" means

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Inapurelybiologicalsensefearbeginswiththebody’ssystemforreactingtothingsthatcanharmus—theso-calledfight-or-flightresponse.Ananimalthatcan’tdetectdangercan’tstayalivesaysJosephLeDoux.Likeanimalshumansevolvedwithanelaboratemechanismforprocessinginformationaboutpotentialthreats.Atitscoreisaclusterofneuronsdeepinthebrainknownastheamygdala. LeDouxstudiesthewayanimalsandhumansrespondtothreatstounderstandhowweformmemoriesofsignificanteventsinourlives.Theamygdalareceivesinputfrommanypartsofthebrainincludingregionsresponsibleforretrievingmemories.Usingthisinformationtheamygdalaappraisesasituation—Ithinkthischargingdogwantstobiteme—andtriggersaresponsebyradiatingnervesignalsthroughoutthebody.Thesesignalsproducethefamiliarsignsofdistress:tremblingperspirationandfast-movingfeetjusttonamethree. Thisfearmechanismiscriticaltothesurvivalofallanimalsbutnoonecansayforsurewhetherbeastsotherthanhumansknowthey’reafraid.ThatisasLeDouxsaysifyouputthatsystemintoabrainthathasconsciousnessthenyougetthefeelingoffear. HumanssaysEdwardM.Hallowellhavetheabilitytocallupimagesofbadthingsthathappenedinthepastandtoanticipatefutureevents.Combinethesehigherthoughtprocesseswithourhardwireddangerdetectionsystemsandyougetanear-universalhumanphenomenon:worry. That’snotnecessarilyabadthingsaysHallowell.Whenusedproperlyworryisanincredibledevicehesays.Afterallalittlehealthyworryingisokayifitleadstoconstructiveaction—likehavingadoctorlookatthatweirdspotonyourback. Hallowellinsiststhoughthatthere’sarightwaytoworry.Neverdoitalonegetthefactsandthenmakeaplan.Hesays.Mostofushavesurvivedarecessionsowe’refamiliarwiththebelt-tighteningstrategiesneededtosurviveaslump. Unfortunatelyfewofushavemuchexperiencedealingwiththethreatofterrorismsoit’sbeendifficulttogetfactabouthowweshouldrespond.That’swhyHallowellbelievesitwasokayforpeopletoindulgesomeextremeworrieslastfallbyaskingdoctorsforCiproandbuyinggasmasks. WhichofthefollowingisthebestwaytodealWithyourworriesaccordingtoHallowell
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设fx+yxy=x2+y2+xy则dfxy=______.
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设幂级数的收敛半径为2则级数a+1n的收敛区间为______
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设函数fx连续则=______
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Chronicinsomniaisamajorpublichealthproblem.Andtoomanypeopleareusing1therapiesevenwhilethereareafewtreatmentsthatdowork.MillionsofAmericans2awakeatnightcountingsheeporhaveastiffdrinkor3anpillhopingitwillmakethemsleepy.4expertsagreeallthatself-medicatingisabadideaandthecausesofchronicinsomniaremain5.a Chronicinsomniaisamajorpublichealthproblem.Andtoomanypeopleareusing1therapiesevenwhilethereareafewtreatmentsthatdowork.MillionsofAmericans2awakeatnightcountingsheeporhaveastiffdrinkor3anpillhopingitwillmakethemsleepy.4expertsagreeallthatself-medicatingisabadideaandthecausesofchronicinsomniaremain5. Almostathirdofadultshavetroublesleepingandabout10percenthave6ofdaytimeimpairmentthatsignaltrueinsomnia.But7thecomplaintsscientistsknowsurprisinglylittleaboutwhatcauseschronicinsomniaitshealthconsequencesandhowbesttotreatitapanelofspecialists8togetherbytheNationalInstitutesofHealthconcludedWednesday.Thepanelcalled9abroadrangeofresearchintoinsomnia10thatifscientistsunderstoodits11causestheycoulddevelopbettertreatments. Mostbutnotallinsomniaisthoughtto12otherhealthproblemsfromarthritisanddepressiontocardiovasculardisease.Thequestionofteniswhethertheinsomniacamefirstorwasaresultoftheotherdiseasesandhowtroublesleepingin13complicatesthoseotherproblems.Otherdiseases14theriskofinsomniaseemstoincreasewithageandtobemore15amongwomenespeciallyaftertheir50s.Smokingcaffeineandnumerous16drugsalsoaffectsleep. TheNIHisspendingabout$200millionthisyearonsleep-relatedresearchsome17tospecificdisordersandothers18theunderlyingscientificlawsthatcontrolthenervoussystemofsleep.Theagencywas19thepane’sreviewbeforedecidingwhatadditionalworkshouldbe20atinsomnia. 17
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Chronicinsomniaisamajorpublichealthproblem.Andtoomanypeopleareusing1therapiesevenwhilethereareafewtreatmentsthatdowork.MillionsofAmericans2awakeatnightcountingsheeporhaveastiffdrinkor3anpillhopingitwillmakethemsleepy.4expertsagreeallthatself-medicatingisabadideaandthecausesofchronicinsomniaremain5.a Chronicinsomniaisamajorpublichealthproblem.Andtoomanypeopleareusing1therapiesevenwhilethereareafewtreatmentsthatdowork.MillionsofAmericans2awakeatnightcountingsheeporhaveastiffdrinkor3anpillhopingitwillmakethemsleepy.4expertsagreeallthatself-medicatingisabadideaandthecausesofchronicinsomniaremain5. Almostathirdofadultshavetroublesleepingandabout10percenthave6ofdaytimeimpairmentthatsignaltrueinsomnia.But7thecomplaintsscientistsknowsurprisinglylittleaboutwhatcauseschronicinsomniaitshealthconsequencesandhowbesttotreatitapanelofspecialists8togetherbytheNationalInstitutesofHealthconcludedWednesday.Thepanelcalled9abroadrangeofresearchintoinsomnia10thatifscientistsunderstoodits11causestheycoulddevelopbettertreatments. Mostbutnotallinsomniaisthoughtto12otherhealthproblemsfromarthritisanddepressiontocardiovasculardisease.Thequestionofteniswhethertheinsomniacamefirstorwasaresultoftheotherdiseasesandhowtroublesleepingin13complicatesthoseotherproblems.Otherdiseases14theriskofinsomniaseemstoincreasewithageandtobemore15amongwomenespeciallyaftertheir50s.Smokingcaffeineandnumerous16drugsalsoaffectsleep. TheNIHisspendingabout$200millionthisyearonsleep-relatedresearchsome17tospecificdisordersandothers18theunderlyingscientificlawsthatcontrolthenervoussystemofsleep.Theagencywas19thepane’sreviewbeforedecidingwhatadditionalworkshouldbe20atinsomnia. 11
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A是n阶矩阵且A3=0则______
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设y=1+sinxx则=______
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设二维随机变量XY在区域D://0<x<1|y|=x内服从均均分布求关于X的边缘概率密度函数及随机变量Z=2X+1的方差DZ
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设fx为连续函数计算其中D是由y=x3y=1z=-1围成的区域
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设A为n阶方阵A*为A的伴随矩阵且A11≠0证明方程组Ax=bb≠0有无穷多解的充要条件中b为A*X=0的解
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- =______.
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- Chronicinsomniaisamajorpublichealthproblem.Andtoomanypeopleareusing1therapiesevenwhilethereareafewtreatmentsthatdowork.MillionsofAmericans2awakeatnightcountingsheeporhaveastiffdrinkor3anpillhopingitwillmakethemsleepy.4expertsagreeallthatself-medicatingisabadideaandthecausesofchronicinsomniaremain5.a Chronicinsomniaisamajorpublichealthproblem.Andtoomanypeopleareusing1therapiesevenwhilethereareafewtreatmentsthatdowork.MillionsofAmericans2awakeatnightcountingsheeporhaveastiffdrinkor3anpillhopingitwillmakethemsleepy.4expertsagreeallthatself-medicatingisabadideaandthecausesofchronicinsomniaremain5. Almostathirdofadultshavetroublesleepingandabout10percenthave6ofdaytimeimpairmentthatsignaltrueinsomnia.But7thecomplaintsscientistsknowsurprisinglylittleaboutwhatcauseschronicinsomniaitshealthconsequencesandhowbesttotreatitapanelofspecialists8togetherbytheNationalInstitutesofHealthconcludedWednesday.Thepanelcalled9abroadrangeofresearchintoinsomnia10thatifscientistsunderstoodits11causestheycoulddevelopbettertreatments. Mostbutnotallinsomniaisthoughtto12otherhealthproblemsfromarthritisanddepressiontocardiovasculardisease.Thequestionofteniswhethertheinsomniacamefirstorwasaresultoftheotherdiseasesandhowtroublesleepingin13complicatesthoseotherproblems.Otherdiseases14theriskofinsomniaseemstoincreasewithageandtobemore15amongwomenespeciallyaftertheir50s.Smokingcaffeineandnumerous16drugsalsoaffectsleep. TheNIHisspendingabout$200millionthisyearonsleep-relatedresearchsome17tospecificdisordersandothers18theunderlyingscientificlawsthatcontrolthenervoussystemofsleep.Theagencywas19thepane’sreviewbeforedecidingwhatadditionalworkshouldbe20atinsomnia. 19
- 函数y=在区间[02]上的平均值为______.
- 设fx在[ab]上连续在ab内二阶可导又设连接afabfb两点的直线和曲线y=fx相交于点cfca<c<b求证在ab内至少存在一点ξ使得fξ=0
- 设X1Xn是取自总体X一个简单随机样本X的概率密度为Ⅰ求未知参数θ的矩估计量Ⅱ求未知参数θ的最大似然估计量
- 设yn是n次伯努利试验中事件A出现的次数p为A在每次试验中出现的概率则对任意ε>0有=______
- 求幂级数的收敛域及和函数
- 设A为nn≥2阶可逆方阵交换A的第1行与第2行得矩阵BA*B*分别是AB的伴随矩阵则______
- 设fx在闭区间[ab]上连续开区间ab内可导证明在区间ab内至少存在两点ξ1ξ2使f’ξ2tan
- 设XY~fxy=求联合分布函数Fxy
- 已知函数fx具有任意阶导数且f’x=[fx]2则当n为大于2的正整数时fx的n阶导数fnx为______
- 幂级数的收敛域为______
- 设矩阵A=E+2αβT其中αβ是n维列向量且αTβ=2则A-1=______
- 设z=zxy由方程xy=xfz+ygz确定且xf’z+yg’z≠0则[x-gz]-=______
- 设X1X2Xn相互独立则X1X2Xn满足辛钦大数定律的条件是______
- 已知二维向量α不是二阶方阵A的特征向量Ⅰ证明αAα线性无关Ⅱ若A2α+Aα-6α=0求A的全部特征值并判断A能否与对角矩阵相似
- 设矩阵A与B=相似则rA.+rA-2E=______
- 设向量组α1α2α3β1线性相关向量组α1α2α3β2线性无关则对于任意常数k必有______.
- 求解差方方程3yx+1-9yx=x·3x+1.
- 已知XY的联合概率密度为X与Y是否相互独立并说明理由
- 已知随机变量X的概率分布为k=123当X=k时随机变量Y在0k上服从均匀分布即则PY≤2.5=______
- 设=1则函数fx在点x=a处______
- 设随机变量X和Y相互独立且X和Y的概率分布分别为则pX+Y=2=______
- 方程x4+4x+b=0有两个不等的实根则b的取值满足______
- Upuntilafewdecadesagoourvisionsofthefuturewerelargely—thoughbynomeansuniformly—glowinglypositive.Scienceandtechnologywouldcurealltheillsofhumanityleadingtolivesoffulfillmentandopportunityforall.Nowutopiahasgrownunfashionableaswehavegainedadeeperappreciationoftherangeofthreatsfacingusfromasteroidstriketoepidemicflutoclimatechange.Youmightevenbetemptedtoassumethathumanityhaslittlefuturetolookforwardto.Butsuchgloominessismisplaced.Thefossilrecordshowsthatmanyspecieshaveenduredformillionsofyears—sowhyshouldn’tweTakeabroaderlookatourspecies’placeintheuniverseanditbecomesclearthatwehaveanexcellentchanceofsurvivingfortensifnothundredsofthousandsofyears.LookupHomosapiensintheRedListofthreatenedspeciesoftheInternationalUnionfortheConversationofNatureIUCNandyouwillread:ListedasLeastConcernasthespeciesisverywidelydistributedadaptablecurrentlyincreasingandtherearenomajorthreatsresultinginanoverallpopulationdecline.SowhatdoesourdeepfutureholdAgrowingnumberofresearchersandorganizationsarenowthinkingseriouslyaboutthatquestion.ForexampletheLongNowFoundationhasitsflagshipprojectamechanicalclockthatisdesignedtostillbemarkingtimethousandsofyearshence.Perhapswillfullyitmaybeeasiertothinkaboutsuchlengthytimescalesthanaboutthemoreimmediatefuture.Thepotentialevolutionoftoday’stechnologyanditssocialconsequencesisdazzlinglycomplicatedandit’sperhapsbestlefttosciencefictionwritersandfuturologiststoexplorethemanypossibilitieswecanenvisage.That’sonereasonwhywehavelaunchedArcanewpublicationdedicatedtothenearfuture.Buttakealongerviewandthereisasurprisingamountthatwecansaywithconsiderableassurance.Assooftenthepastholdsthekeytothefuture:wehavenowidentifiedenoughofthelong-termpatternsshapingthehistoryoftheplanetandourspeciestomakeevidence-basedforecastsaboutthesituationsinwhichourdescendantswillfindthemselves.Thislongperspectivemakesthepessimisticviewofourprospectsseemmorelikelytobeapassingfad.Tobesurethefutureisnotallrosy.Butwearenowknowledgeableenoughtoreducemanyoftherisksthatthreatenedtheexistenceofearlierhumansandtoimprovethelotofthosetocome.Whichofthefollowingwouldbethebesttitleforthetext
- 设总体X服从正态分布N0σ2σ2已知X1X2Xn是取自总体X的简单随机样本S2为样本方差则______.
- 设函数fx在|x|≤1上有定义在x=0的某个邻域内具有二阶连续导数且=0试证级数绝对收敛
- 求
- 设A为三阶实对称矩阵λ1=8λ2=λ3=2是其特征值.已知对应λ1=8的特征向量为α1=[1k1]T对应λ2=λ3=2的一个特征向量为α2=[-110]T.试求参数k及λ2-λ3=2的另一个特征向量和矩阵A.
- 设z=u2v3v=e2xsinyv=x2+y3求
- 设f00=0当xy≠00时fxy为如下形式之一则fxy在00处可偏导的是______
- 若=+∞则级数______
- 设二次型经过正交变换X=Qy化为标准形求参数ab及正交矩阵Q
- 设四维向量组α1=1142Tα2=1-1-2bTα3=-3-1α-9Tβ=1310a+bT[问]Ⅰ当ab取何值时β不能由α1α2α3线性表出Ⅱ当ab取何值时β能由α1α2α3线性表出并写出此时的表达式
- 设f有二阶连续偏导数u=fxxyxyz则=______
- LongbeforeManlivedontheEarththerewerefishesreptilesbirdsinsectsandsomemammals.Althoughsomeoftheseanimalswereancestorsofkindslivingtodayothersarenowextinctthatistheyhavenodescendantsalivenow.41Veryoccasionallytherocksshowimpressionofskinsothatapartfromcolorwecanbuildupareasonablyaccuratepictureofananimalthatdiedmillionsofyearsago.Thatkindofrockinwhichtheremainsarefoundtellsusmuchaboutthenatureoftheoriginallandoftenoftheplantsthatgrewonitandevenofitsclimate.42Nearlyallofthefossilsthatweknowwerepreservedinrocksformedbywateractionandmostoftheseareofanimalsthatlivedinornearwater.Thusitfollowsthattheremustbemanykindsofmammalsbirdsandinsectsofwhichweknownothing.43Therewerealsocrablikecreatureswhosebodieswerecoveredwithahornysubstance.Thebodysegmentseachhadtwopairsoflegsonepairforwalkingonthesandybottomtheotherforswimming.Theheadwasakindofshieldwithapairofcompoundeyesoftenwiththousandsoflenses.Theywereusuallyaninchortwolongbutsomewere2feet.44Ofthesetheammonitesareveryinterestingandimportant.Theyhaveashellcomposedofmanychamberseachrepresentingatemporaryhomeoftheanimal.Astheyounggrewlargeritgrewanewchamberandsealedoffthepreviousone.ThousandsofthesecanbeseenintherocksontheDorsetCoast.45About75millionyearsagotheAgeofReptileswasoverandmostofthegroupsdiedout.Themammalsquicklydevelopedandwecantracetheevolutionofmanyfamiliaranimalssuchastheelephantandhorse.Manyofthelatermammalsthoughnowextinctwereknowntoprimitivemanandwerefeaturedbyhimincavepaintingsandonbonecarvings.A.Theshellfishhavealonghistoryintherockandmanydifferentkindsareknown.B.Neverthelessweknowagreatdealaboutmanyofthembecausetheirbonesandshellshavebeenpreservedintherocksasfossilsfromthemwecantelltheirsizeandshapehowtheywalkedthekindoffoodtheyate.C.Thefirstanimalswithtruebackboneswerethefishesfirstknownintherocksof375millionyearsago.About300millionyearsagotheamphibianstheanimalsabletolivebothonlandandinwaterappeared.Theyweregiantsometimes8feetlongandmanyofthemlivedintheswampypoolsinwhichourcoalseamorlayerisformed.Theamphibiansgaverisetothereptilesandfornearly150millionyearstheseweretheprincipalformsoflifeonlandintheseaandintheair.D.Thebestindexfossilstendtobemarinecreatures.Theseanimalsevolvedrapidlyandspreadoverlargeoverlargeareasoftheworld.E.Theearliestanimalswhoseremainshavebeenfoundwereallverysimplekindsandlivedinthesea.Laterformsaremorecomplexandamongthesearethesea-liliesrelationsofthestar-fisheswhichhadlongarmsandwereattachedbyalongstalktotheseabedortorocks.F.Whenananimaldiesthebodyitsbonesorshellmayoftenbecarriedawaybystreamsintolakesortheseaandthengetcoveredupbymud.Iftheanimallivedintheseaitsbodywouldprobablysinkandbecoveredwithmud.Moreandmoremudwouldfalluponituntilthebonesorshellbecomeembeddedandpreserved.G.Manyfactorscaninfluencehowfossilsarepreservedinrocks.Remainsofanorganismmaybereplacedbymineralsdissolvedbyanacidicsolutiontoleaveonlytheirimpressionorsimplyreducedtoamorestableform.
- 设fx在[01]上连续在01内可导f0=0当x>0时fx>0.证明对任意自然数k存在ξ∈01使
- 设n阶矩阵A=α1α2αn的前n-1个列向量线性相关后n-1个列向量线性无关且α1+α2++n-1αn-1=0b=α1+α2++αnⅠ证明方程组AX=b有无穷多个解Ⅱ求方程组AX=b的通解
- 设fx在-∞+∞内连续且严格单调增f0=0常数n为正奇数并设Fx=则下列判断正确的是______
- 设z=f2x-yysinx其中fuv具有连续的二阶偏导数求
- 设fx=n∈N若与都存在那么a+b=______
- 设有2个四元齐次线性方程组ⅠⅡ方程组Ⅰ和Ⅱ是否有非零公共解若有求出所有的非零公共解若没有则说明理由.
- 保险公司为50个集体投保人提供医疗保险.假设他们医疗花费相互独立且花费单位百元服从相同的分布律当花费超过一百元时保险公司应支付超过百元的部分当花费不超过一百元时由患者自己负担费用.如果以总支付费X的期望值EX作为预期的总支付费那么保险公司应收取总保险费为1+θEX其中θ为相对附加保险费.为使公司获利的概率超过95%附加保险费θ至少应为多少已知Φ1.41=0.92Φ1.62=0.95
- 设fx在点x=a可导则极限______=f’a.
- 已知二次型fx1x2x3=经正交变换化为标准形fx1x2x3=则ab取值为______.
- 46AnydiscussionoftheAmericaneducationalsystemwouldbelessthancompleteifitdidnotmentiontheemphasisthatmanycollegesanduniversitiesplaceuponthenonacademicsocialextracurricularaspectofeducationoftendefinedaspersonalgrowth.Perhapsausefulwayofviewingthenotionofpersonalgrowthwouldbetopicturetheverylargeandgeneraltermeducationasbeingall-embracingincludingassubsetswithinitacademicandnonacademiccomponents.ThismaybeoneofthemostdifficultconceptstoconveytosomeonewhoisnotintimatelyfamiliarwithAmericanhighereducation.Feweducationalsystemsinothercountriesplacethesameemphasisonthisblendofacademicandpersonaleducation.ThemajorityofcollegesanduniversitiesintheUnitedStatesmakesomeattempttointegratepersonalandintellectualgrowthintheundergraduateyears.47IftheultimategoalofundergraduateeducationinAmericaweresimplytoconveyasetbodyofknowledgethetermofstudiescouldundoubtedlybereduced.Yetthetermsofstudiesareextendedinordertogivestudentsachancetogrowanddevelopinotherways.Numerousopportunitiesaremadeavailabletostudentstobecomeinvolvedinsportsstudentgovernmentmusicalanddramaticorganizationsandcountlessotherorganizedandindividualactivitiesdesignedtoenhanceone’spersonalgrowthandprovidesomerecreationandenjoymentoutsideoftheclassroom.48Experiencewithcampusorganizationsandoff-campuscommunityinvolvementcanbehighlyvaluableinpreparinginternationalstudentsforfutureleadershipintheirprofessionalfieldupontheirreturnhome.ThetypicalAmericancollege’ssupportforextracurricularactivityisperhapsuniqueintheworld.ThisspecialeducationaldimensionbeyondtheclassroomandlaboratoryexperiencedoesnotmeanthatextracurricularparticipationisrequiredtogainanAmericandegree.Itremainsanentirelyoptionalactivitybut49itisnotedherebecauseAmericanshavetraditionallyviewedsuccessinone’sroleasacitizenascloselylinkedtoawell-roundedlifethatincorporatesavarietyofsocialathleticandculturalactivitiesintoaperson’sexperience.AgreatmanyAmericancampusesandcommunitieshaveorganizedspecialextracurricularactivitiesforstudentsfromothercountries.50OnmostcampusesonecanfindaninternationalclubwhichincludesAmericanswherestudentscangettoknowandlearnsociallyfromstudentsfromothercountriesaswellasAmericans.InternationalstudentsarealmostalwaysinvitedthroughorganizedhospitalityactivitiesintothehomesofAmericanslivinginoroutsidetheacademiccommunity.
- 若函数y=fx有f’x0=则当Δx→0时该函数在x=x0点外的微分dy是______
- 设fx在[ab]上连续且fx>0又证明F’x≥2
- LongbeforeManlivedontheEarththerewerefishesreptilesbirdsinsectsandsomemammals.Althoughsomeoftheseanimalswereancestorsofkindslivingtodayothersarenowextinctthatistheyhavenodescendantsalivenow.41Veryoccasionallytherocksshowimpressionofskinsothatapartfromcolorwecanbuildupareasonablyaccuratepictureofananimalthatdiedmillionsofyearsago.Thatkindofrockinwhichtheremainsarefoundtellsusmuchaboutthenatureoftheoriginallandoftenoftheplantsthatgrewonitandevenofitsclimate.42Nearlyallofthefossilsthatweknowwerepreservedinrocksformedbywateractionandmostoftheseareofanimalsthatlivedinornearwater.Thusitfollowsthattheremustbemanykindsofmammalsbirdsandinsectsofwhichweknownothing.43Therewerealsocrablikecreatureswhosebodieswerecoveredwithahornysubstance.Thebodysegmentseachhadtwopairsoflegsonepairforwalkingonthesandybottomtheotherforswimming.Theheadwasakindofshieldwithapairofcompoundeyesoftenwiththousandsoflenses.Theywereusuallyaninchortwolongbutsomewere2feet.44Ofthesetheammonitesareveryinterestingandimportant.Theyhaveashellcomposedofmanychamberseachrepresentingatemporaryhomeoftheanimal.Astheyounggrewlargeritgrewanewchamberandsealedoffthepreviousone.ThousandsofthesecanbeseenintherocksontheDorsetCoast.45About75millionyearsagotheAgeofReptileswasoverandmostofthegroupsdiedout.Themammalsquicklydevelopedandwecantracetheevolutionofmanyfamiliaranimalssuchastheelephantandhorse.Manyofthelatermammalsthoughnowextinctwereknowntoprimitivemanandwerefeaturedbyhimincavepaintingsandonbonecarvings.A.Theshellfishhavealonghistoryintherockandmanydifferentkindsareknown.B.Neverthelessweknowagreatdealaboutmanyofthembecausetheirbonesandshellshavebeenpreservedintherocksasfossilsfromthemwecantelltheirsizeandshapehowtheywalkedthekindoffoodtheyate.C.Thefirstanimalswithtruebackboneswerethefishesfirstknownintherocksof375millionyearsago.About300millionyearsagotheamphibianstheanimalsabletolivebothonlandandinwaterappeared.Theyweregiantsometimes8feetlongandmanyofthemlivedintheswampypoolsinwhichourcoalseamorlayerisformed.Theamphibiansgaverisetothereptilesandfornearly150millionyearstheseweretheprincipalformsoflifeonlandintheseaandintheair.D.Thebestindexfossilstendtobemarinecreatures.Theseanimalsevolvedrapidlyandspreadoverlargeoverlargeareasoftheworld.E.Theearliestanimalswhoseremainshavebeenfoundwereallverysimplekindsandlivedinthesea.Laterformsaremorecomplexandamongthesearethesea-liliesrelationsofthestar-fisheswhichhadlongarmsandwereattachedbyalongstalktotheseabedortorocks.F.Whenananimaldiesthebodyitsbonesorshellmayoftenbecarriedawaybystreamsintolakesortheseaandthengetcoveredupbymud.Iftheanimallivedintheseaitsbodywouldprobablysinkandbecoveredwithmud.Moreandmoremudwouldfalluponituntilthebonesorshellbecomeembeddedandpreserved.G.Manyfactorscaninfluencehowfossilsarepreservedinrocks.Remainsofanorganismmaybereplacedbymineralsdissolvedbyanacidicsolutiontoleaveonlytheirimpressionorsimplyreducedtoamorestableform.
- 计算
- Sustainabledevelopmentisappliedtojustabouteverythingfromenergytocleanwaterandeconomicgrowthandasaresultithasbecomedifficulttoquestioneitherthebasicassumptionsbehinditorthewaytheconceptisputtouse.Thisisespeciallytrueinagriculturewheresustainabledevelopmentisoftentakenasthesolemeasureofprogresswithoutaproperappreciationofhistoricalandculturalperspectives.Tostartwithitisimportanttorememberthatthenatureofagriculturehaschangedmarkedlythroughouthistoryandwillcontinuetodoso.MedievalagricultureinnorthernEuropefedclothedandshelteredapredominantlyruralsocietywithamuchlowerpopulationdensitythanitistoday.Ithadminimaleffectonbiodiversityandanypollutionitcausedwastypicallylocalized.Intermsofenergyuseandthenutrientscapturedintheproductitwasrelativelyinefficient.Contrastthiswithfarmingsincethestartoftheindustrialrevolution.Competitionfromoverseasledfarmerstospecializeandincreaseyields.Throughoutthisperiodfoodbecamecheapersafeandmorereliable.Howeverthesechangeshavealsoledtohabitatlossandtodiminishingbiodiversity.What’smoredemandforanimalproductsindevelopingcountriesisgrowingsofastthatmeetingitwillrequireanextra300milliontonsofgrainayearby2050.Yetthegrowthofcitiesandindustryisreducingtheamountofwateravailableforagricultureinmanyregions.Allthismeansthatagricultureinthe21stcenturywillhavetobeverydifferentfromhowitwasinthe20th.Thiswillrequireradicalthinking.Forexampleweneedtomoveawayfromtheideathattraditionalpracticesareinevitablymoresustainablethannewones.Wealsoneedtoabandonthenotionthatagriculturecanbezeroimpact.Thekeywillbetoabandontherathersimpleandstaticmeasuresofsustainabilitywhichcentreontheneedtomaintainproductionwithoutincreasingdamage.Insteadweneedamoredynamicinterpretationonethatlooksattheprosandconsofallthevariouswaylandisused.Therearemanydifferentwaystomeasureagriculturalperformancebesidesfoodyield:energyuseenvironmentalcostswaterpuritycarbonfootprintandbiodiversity.ItisclearforexamplethatthecarbonoftransportingtomatoesfromSpaintotheUKislessthanthatofproducingthemintheUKwithadditionalheatingandlighting.Butwedonotknowwhetherlowercarbonfootprintswillalwaysbebetterforbiodiversity.Whatiscrucialisrecognizingthatsustainableagricultureisnotjustaboutsustainablefoodproduction.Whatwillagriculturebelikeinthe21stcentury
- 设A为n阶矩阵其伴随矩阵的元素全为1则齐次方程组Ax=0的通解为______
- 设那么与对角矩阵相似的矩阵是______.
- 假设二维连续型随机变量XY在矩形区域D=xy|0≤x≤20≤y≤1上服从均匀分布令Z=maxXY求EZ与DZ
- 设二维随机变量XY服从二维正态分布则随机变量ξ=X+Y与n=X-Y不相关的充分必要条件为______.
- 设随机变量X与Y服从正态分布X~Nμ42Y~Nμ52记P1=Px≤μ-4P2=Y≥μ+5则______
- 设fx在点x=0处可导且则f’0=______
- 设D=xy|x2+y2≤R2R>0常数λ≠0则积分的值______
- 计算其中D:0≤x≤y≤2π
- 设fx+1=afx总成立f’0=bab为非零常数则fx在x=1处______
- 设随机变量X与Y分别服从N-12和N12且X与Y不相关k1X+Y与X+k2Y也不相关则______
- 求微分方程Y+5y’+6y=2e-x的通解
- 设I=xydxdy其中D由曲线y=y=-x和y=所围成则I的值为______.
- LongbeforeManlivedontheEarththerewerefishesreptilesbirdsinsectsandsomemammals.Althoughsomeoftheseanimalswereancestorsofkindslivingtodayothersarenowextinctthatistheyhavenodescendantsalivenow.41Veryoccasionallytherocksshowimpressionofskinsothatapartfromcolorwecanbuildupareasonablyaccuratepictureofananimalthatdiedmillionsofyearsago.Thatkindofrockinwhichtheremainsarefoundtellsusmuchaboutthenatureoftheoriginallandoftenoftheplantsthatgrewonitandevenofitsclimate.42Nearlyallofthefossilsthatweknowwerepreservedinrocksformedbywateractionandmostoftheseareofanimalsthatlivedinornearwater.Thusitfollowsthattheremustbemanykindsofmammalsbirdsandinsectsofwhichweknownothing.43Therewerealsocrablikecreatureswhosebodieswerecoveredwithahornysubstance.Thebodysegmentseachhadtwopairsoflegsonepairforwalkingonthesandybottomtheotherforswimming.Theheadwasakindofshieldwithapairofcompoundeyesoftenwiththousandsoflenses.Theywereusuallyaninchortwolongbutsomewere2feet.44Ofthesetheammonitesareveryinterestingandimportant.Theyhaveashellcomposedofmanychamberseachrepresentingatemporaryhomeoftheanimal.Astheyounggrewlargeritgrewanewchamberandsealedoffthepreviousone.ThousandsofthesecanbeseenintherocksontheDorsetCoast.45About75millionyearsagotheAgeofReptileswasoverandmostofthegroupsdiedout.Themammalsquicklydevelopedandwecantracetheevolutionofmanyfamiliaranimalssuchastheelephantandhorse.Manyofthelatermammalsthoughnowextinctwereknowntoprimitivemanandwerefeaturedbyhimincavepaintingsandonbonecarvings.A.Theshellfishhavealonghistoryintherockandmanydifferentkindsareknown.B.Neverthelessweknowagreatdealaboutmanyofthembecausetheirbonesandshellshavebeenpreservedintherocksasfossilsfromthemwecantelltheirsizeandshapehowtheywalkedthekindoffoodtheyate.C.Thefirstanimalswithtruebackboneswerethefishesfirstknownintherocksof375millionyearsago.About300millionyearsagotheamphibianstheanimalsabletolivebothonlandandinwaterappeared.Theyweregiantsometimes8feetlongandmanyofthemlivedintheswampypoolsinwhichourcoalseamorlayerisformed.Theamphibiansgaverisetothereptilesandfornearly150millionyearstheseweretheprincipalformsoflifeonlandintheseaandintheair.D.Thebestindexfossilstendtobemarinecreatures.Theseanimalsevolvedrapidlyandspreadoverlargeoverlargeareasoftheworld.E.Theearliestanimalswhoseremainshavebeenfoundwereallverysimplekindsandlivedinthesea.Laterformsaremorecomplexandamongthesearethesea-liliesrelationsofthestar-fisheswhichhadlongarmsandwereattachedbyalongstalktotheseabedortorocks.F.Whenananimaldiesthebodyitsbonesorshellmayoftenbecarriedawaybystreamsintolakesortheseaandthengetcoveredupbymud.Iftheanimallivedintheseaitsbodywouldprobablysinkandbecoveredwithmud.Moreandmoremudwouldfalluponituntilthebonesorshellbecomeembeddedandpreserved.G.Manyfactorscaninfluencehowfossilsarepreservedinrocks.Remainsofanorganismmaybereplacedbymineralsdissolvedbyanacidicsolutiontoleaveonlytheirimpressionorsimplyreducedtoamorestableform.
- 设函数z=zxy是由方程z=2y+e2x-3z确定则______
- 46AnydiscussionoftheAmericaneducationalsystemwouldbelessthancompleteifitdidnotmentiontheemphasisthatmanycollegesanduniversitiesplaceuponthenonacademicsocialextracurricularaspectofeducationoftendefinedaspersonalgrowth.Perhapsausefulwayofviewingthenotionofpersonalgrowthwouldbetopicturetheverylargeandgeneraltermeducationasbeingall-embracingincludingassubsetswithinitacademicandnonacademiccomponents.ThismaybeoneofthemostdifficultconceptstoconveytosomeonewhoisnotintimatelyfamiliarwithAmericanhighereducation.Feweducationalsystemsinothercountriesplacethesameemphasisonthisblendofacademicandpersonaleducation.ThemajorityofcollegesanduniversitiesintheUnitedStatesmakesomeattempttointegratepersonalandintellectualgrowthintheundergraduateyears.47IftheultimategoalofundergraduateeducationinAmericaweresimplytoconveyasetbodyofknowledgethetermofstudiescouldundoubtedlybereduced.Yetthetermsofstudiesareextendedinordertogivestudentsachancetogrowanddevelopinotherways.Numerousopportunitiesaremadeavailabletostudentstobecomeinvolvedinsportsstudentgovernmentmusicalanddramaticorganizationsandcountlessotherorganizedandindividualactivitiesdesignedtoenhanceone’spersonalgrowthandprovidesomerecreationandenjoymentoutsideoftheclassroom.48Experiencewithcampusorganizationsandoff-campuscommunityinvolvementcanbehighlyvaluableinpreparinginternationalstudentsforfutureleadershipintheirprofessionalfieldupontheirreturnhome.ThetypicalAmericancollege’ssupportforextracurricularactivityisperhapsuniqueintheworld.ThisspecialeducationaldimensionbeyondtheclassroomandlaboratoryexperiencedoesnotmeanthatextracurricularparticipationisrequiredtogainanAmericandegree.Itremainsanentirelyoptionalactivitybut49itisnotedherebecauseAmericanshavetraditionallyviewedsuccessinone’sroleasacitizenascloselylinkedtoawell-roundedlifethatincorporatesavarietyofsocialathleticandculturalactivitiesintoaperson’sexperience.AgreatmanyAmericancampusesandcommunitieshaveorganizedspecialextracurricularactivitiesforstudentsfromothercountries.50OnmostcampusesonecanfindaninternationalclubwhichincludesAmericanswherestudentscangettoknowandlearnsociallyfromstudentsfromothercountriesaswellasAmericans.InternationalstudentsarealmostalwaysinvitedthroughorganizedhospitalityactivitiesintothehomesofAmericanslivinginoroutsidetheacademiccommunity.