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Cos2A除以CosA

因为cos2a=cosacosa-sinasina=(cosa-sina)(cosa+sina),所以,cos2a/(cosa-sina)=cosa+sina

倍角公式学过么?cos 2a = cosa^2-sina^2又有1 = cosa^2+sina^2因此原式等于:2sina^2/sinacosa等于2tana

公式cos2a=2cosa-1 cos2a=cos(a+a)=cosacosa-sinacosa=cosa-sina=cosa-(1-cosa)=2cosa-1

tan2a

cos2a=1-cos^2

【1】和差化积.cos(2A)-cosA=cos(1.5A+0.5A)-cos(1.5A-0.5A)=[cos(1.5A)cos(0.5A)-sin(1.5A)sin(0.5A)] -[cos(1.5A)cos(0.5A)+sin(1.5A)sin(0.5A)]=-2sin(1.5A)sin(0.5A)【2】求极值.f(A)=cos(2A)-cosA=2cosA-1-cosA=2(cosA-1/4)-9/8.最小值.当cosA=1/4时,f(A)=-9/8.最大值.当cosA=-1时,f(A)=2(-1-1/4)-9/8=2.由于函数可导,最值也是极值.温馨提示:数学题,需要说明【具体】做什么.

sinx/cosx=tanx这本身就是公式 所以sin2a除以cos2a等于tan2a

解:2sin2a/(1+cos2a)*cosa^2/cos2a =2sin2a/2cosa^2*cosa^2/cos2a =2tana*cosa^2/cos2a =2sina*cosa/cos2a =sin2a/cos2a =tan2a

cos2A=cosA^2-sinA^2=(cosA^2-sinA^2)cosA^2/cosA^2=(1-tanA^2)cosA^2把(1-tanA^2)看成是(1-tanA^2)/(sinA^2+cosA^2),然后在用分母除以cosA^2(分母除以就相当于分子乘以)作者不会是问cos2A=cosA^2-sinA^2?

解:cos2a=cosa^2-sina^2=(cosa+sina)*(cosa-sina) sin(a-π/4)=sina*cos(π/4)-cosa*sin(π/4)=√2(sina-cosa)/2 cos2a 除以 sin(a-pai/4) = 负的2分支根号2 即:(cosa+sina)*(cosa-sina)/[√2(sina-cosa)/2]=-√2/2 故cosa+sina=1/2 选c

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