安徽省霍邱县2023-2024学年度七年级第一学期期中考试数学.考卷答案

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安徽省霍邱县2023-2024学年度七年级第一学期期中考试数学.考卷答案试卷答案

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B.Ifcoded,theinformationwillbefreefromdanger.C.Androidiseasytoattackforlackofencryptionkeys.D.Alloftheinformationm27.WhatcanpcopledotoprotcctagainstKRACKattack?A.Purchasethebest-qualityapps.B.KeeptheproofofKRACKlong.D.InstallthelatestrelevantsoftwarcC.IgnorethepatchesofthecomputersWorkingatadeskalldaymaynotbeasbadforyourhealthassittinginfrontofatelevisionafterwork.ThatisafindingofanewstudydoneintheUnitedStates.Italsosuggeststhatnotallformsofsittingareequallybad.JeanctteGarciaworksasarescarcherattheUniversityofCentralFloridainOrlandoandsheledthestudy."We'vebeenhearingmoreandmoreabouthowsitingisthenewsmoking."shesaid.Sheaddedthatotherevidencesuggestsarelationshipbetweenhealthproblemsandtimespentsitting."However,it'sbeeunclearwhetherallsittingiscreatedequal,suggestingthataslongasyoureduceanytypeofsitting,thenthat'shelpfulinimprovinghealth,"sheadded."Thisstudysuggeststhatthismaynotbethecase,andthatweshouldfocusmoreonleisuretimesitting,mainlytelevisionviewingtime,ratherthanothersitting,ingeneral."Thestudyfollowedcloseto3,600African-Americanadults.AlmostonethirdofthemwatchedTVformorethanfourhoursaday.AlittlemorethanoncthirdofthesubjcetsspentbetweentwoandfourhoursadaysittinginfrontoftheTV.TheremainingthirdwatchedTVforlessthantwohoursaday.Researchersfollowedthegroupforanaverageof8.4years.Duringthestudy,205ofthesubjectsdiedand129otherssufferedaheartattackorstroke(中风)

Comparedtopeoplewhowatchedlessthantwohoursoftelevisiondaily,thosewhospentmorethanfourhoursinfrontoftheTVwere49percentmorelikelytodieorhaveaheartattackorstroke.ButtheundesirablehealtheffectsofwatchingTVappcaredtobelimitedtoadultswhofailedtogettherecommendedamountofweeklyexercise.Thatisaround150minutcsofmildphysicalactivityor75minutesofstrongexerciseeveryweek.Thcstudyhadsomelimitations,however.LinYang,anotherrescarcher,saysthatsittinginfrontoftheTVmightbeworsethansittingatadeskbecausepeoplesnackwhilewatchingTV.Somepeoplesimplystayuptoolateanddonotgetenoughsleep."Eatingtreatsorlackofsleepcouldbothcauseweightgainandincreaseriskfactorsforheartdisease.Thesefactorscouldmakepeoplemorelikelytodieearly,"sheadded.28.WhatcanwelearnfromJeanctteGarcia'sstudy?A.Allformsofsittingarecquallybad.B.Lesssittinghelpstoimprovehealth.C.Sittinglonghasthesameeffectassmoking.D.Healthproblemhasmuchtodowithworkingatadesk.29.Whichofthefollowingwillcausepeopletohavethehighestliferisks?A.WatchingTVlongandsnacking.B.ReadinginthestudyforfourhoursadayC.Failingtogetrecommendedamountofexercise.D.Having75minutesofstrongexerciseeveryweek.30.Whatshouldanadultdotoreducethehighriskoflife?A.Goondiets.B.Takeenoughexercise.

分析（1）利用${C}_{n}^{m}={C}_{n}^{n-m}$和${C}_{n}^{m}+{C}_{n}^{m+1}={C}_{n+1}^{m+1}$，能求出结果．
（2）利用${C}_{n}^{m}+{C}_{n}^{m+1}={C}_{n+1}^{m+1}$，能得到C${\;}_{2}^{2}$+C${\;}_{3}^{2}$+C${\;}_{4}^{2}$+…+C${\;}_{10}^{2}$=${C}_{11}^{3}$，由此能求出结果．

解答解：（1）（C${\;}_{100}^{98}$+C${\;}_{100}^{97}$）÷A${\;}_{101}^{3}$
=（${C}_{100}^{2}+{C}_{100}^{3}$）÷${A}_{101}^{3}$
=${C}_{101}^{3}÷{A}_{101}^{3}$
=$\frac{1}{3！}$
=$\frac{1}{6}$．
（2）C${\;}_{2}^{2}$+C${\;}_{3}^{2}$+C${\;}_{4}^{2}$+…+C${\;}_{10}^{2}$
=${C}_{4}^{3}+{C}_{4}^{2}+{C}_{5}^{2}+…+{C}_{10}^{2}$
=${C}_{5}^{3}+{C}_{5}^{2}+{C}_{6}^{2}+…+{C}_{10}^{2}$
=${C}_{6}^{3}+{C}_{6}^{2}+{C}_{7}^{2}+…+{C}_{10}^{2}$
=${C}_{7}^{3}+{C}_{7}^{2}+{C}_{8}^{2}+…+{C}_{10}^{2}$
=${C}_{8}^{3}+{{C}_{8}^{2}+C}_{9}^{2}+{C}_{10}^{2}$
=${C}_{9}^{3}+{C}_{9}^{2}+{C}_{10}^{2}$
=${C}_{10}^{3}+{C}_{10}^{2}$
=${C}_{11}^{3}$
=$\frac{11×10×9}{3×2×1}$
=165．

点评本题考查组合数公式化简求值，是基础题，解题时要认真审题，注意组合数公式的性质的合理运用．