3，a+b=8，求三角形ABC的面积S
：bcosC+ccosB=2acosCsinBcosC+cosBsinC=2sinAcosCsin（B+C）=2sinAcosCsinA=2sinAcosC∵C∈（0，π）2+b2-c22ab=2-2ab-c22ab=12
菁优解析考点：；．专题：解三角形．分析：（1）已知等式利用正弦定理化简，利用两角和与差的正弦函数公式化简后，根据sinA不为0，求出cosC的值，即可确定出C的度数；（2）利用余弦定理列出关系式，再利用完全平方公式变形后，将c，a+b，cosC的值代入求出ab的值，再利用三角形面积公式即可求出三角形ABC面积．解答：解：（1）已知等式利用正弦定理化简得：sinBcosC+sinCcosB=2sinAcosC，即sin（B+C）=2sinAcosC，变形得：sinA=2sinAcosC，∵sinA≠0，∴cosC=，则∠C=60°；（2）∵c=4，a+b=8，cosC=，∴由余弦定理得：c2=a2+b2-2abcosC=a2+b2-ab=（a+b）2-3ab，即48=64-3ab，整理得：ab=，则S△ABC=absinC=．点评：此题考查了余弦定理，三角形的面积公式，以及特殊角的三角函数值，熟练掌握余弦定理是解本题的关键．答题：sllwyn老师　
其它回答（1条）
1.&&& sin（C+B）=2sinAcosCsin[180-（C+B）]=2sinAcosC&&&&&&&&& sinA=2sinAcosC&&&&&&&&&&&& 1=2cosC&&&&&&&&&&cosC=1/2&&&&&&&&&& ∠C=π/34m(m^-1)=4m(m+1)(m-1)=0--->m=0或±1
-2≤x&-1，或x=1用区间表示为[-2，-1）∪{1}或[-2，-1)∪[1,1]
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he was too tired D? A.he doesn’t need to lose weight,but we went on walking. whatwas the writer’s trouble.He told me that I was too heavy and if I didn’t lose weighr, I had a healthy appetiteI havealways felt proud of my good health because I sometimes only had to take oneday off when I had a cole .When I got back home, Iwould have heart trouble .he has a good doctor.whatcaused his trouble.My doctoradvised me to go on a diet at once.C?A.he was too heavy13. I sat behind my office desk in the day and sat befor therelevision ser in the evening. I would not take my car.he feels he’s in good health.he ate too much C. However.D,either.he had hearttrouble B. I knew we woulddrive about everywhere in it. Iike my food very muchand my wife is a very good cook.12. Therewas no road for car there and we could walk every day.He added that like many middle-aged men.he sometimes takes one day off when he has acoldB.After ourfirst day’s walk I felt very tired.whydoes the writer feel proud, with so much exercise.11. I began to feelhealthier and healthier. I had becomeheavier. I decided that I would not goto the seaside for holiday this year.too easy a life B. We chose a village far away from the city.There you could do nothing but go down tothe beach and lie in the sun.I thendecided that it was all a question of exercise, but failed? A, I lived tooeasy a life.So I was unhappy when I met with my doctor in thestreet one day, I found that I hadn’t lost any weight.I tried
but failed in the end. hetried to lose weight?A. becausethere is no road for car to ger there15C. because it would not give him exercise D.sleeping too much14.becausehe has beento the seaside many times.he tried to eat less.he lost weight with muchexercise.thedoctor advised him to take exercise B. D.too muchtelevision D,but his wife wantedhim to eat moreC. whydid he decide not to go to the seaside for holiday. B. because he would have to take a carthere C.whichof the following is true?A
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When I got back home，没机会锻炼。A答案意思是他有时候感冒请一天假，反而更重了. I would not take my car。C选项作者是在搬到乡村练习步行一天后的感觉. I knew we woulddrive about everywhere in it，所以D不对. I sat behind my office desk in the day and sat befor therelevision ser in the evening。文中最后说However。15。13.这些讲得都是海滩生活会让他终日无所事事，只不过不能，B不对。A选项心脏问题是医生说他如果不减肥将面临的问题。文中没说他妻子不想让他少吃些，所以不是答案，和原意只请一天不同，而运动是他想出的方法，工作太累一般只能让人消瘦，所以C排除：睡眠太多 。因为没能突出是什么值得自豪，所以可看出她是支持他的。B选项。12，他的胃口也更好了. Iike my food very muchand my wife is a very good cook，B排除.说的是运动多了. too easy a life包括too muchtelevision11。14, with so much exercise，所以C排除. We chose a village far away from the city, I had a healthy appetite，而不是现在已存在的问题.选C，因为有时候（即使）得了感冒他也只请一天假。第一句话说I have always felt proud of my good health because I sometimes only had to take oneday off when I had a cold。原文第一段He added that like many middle-aged men，感觉自己很健康而自豪，因为他妻子做的饭太好吃，并且他妻子陪他去乡下减肥，当回家后发现他不但没减肥。D选项. I had becomeheavier。结合全文，他尝试了，医生建议他减肥, Iwould have heart trouble ）。所以答案是B.选B, I lived too easy a life,either, I found that I hadn’t lost any weight.I tried。 原文I decided that I would not goto the seaside for holiday this year，但是失败了。所以这才应该是他的烦恼，超重是现在的状态。第一段他的医生建议他减肥（He told me that I was too heavy and if I didn’t lose weighr.选C，而是说他想少吃，所以A不对.There you could do nothing but go down tothe beach and lie in the sun. 意思是他对自己的健康很自豪。原文My doctoradvised me to go on a diet at once：他吃的太多.选A. Therewas no road for car there and we could walk every day，就是没法控制食物摄入, but failed，文中没有提，而不是现在已存在的问题，所以D也排除，所以和肥胖没什么关系.选B，所以选B。D选项.讲的是医生让他节食。 全文没有对他医生水平做评价
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