sin a sin b +cos a cos b =0,则sin a cos a+sin b cos b的值

由cos(a+b)=cos a cos b-sin a sin b cos(a-b)=cos a cos b+sin a cos(a+b)=cos(a)*cos(b)-sin(a)*sin(b) 用三角形证明 cos^B-cos^C=sin^A,三角形的形状 解三角函数已知 sin(b)+cos(b)=a 求a的值sin(b)*cos(b)=a 证明cos(a+b)=cos(a)cos(b)-sin(a)sin(b)详细步骤,书上的证明看不懂. 若sin^4a/sin^2b+cos^4a/cos^2b=1,证明sin^4b/sin^2a+cos^4b/cos^2a cos(a-b)cos(b-c)+sin(a-b)sin(b-c)= cos(a+b)+cos(a-b)/sin(a+b)+sin(a-b)化简 1.若sin a+sin b=ㄏ2/2,则cos a+cos b的取值范围是多少? 求证:cos(a+b)cos(a-b)=cos平方b-sin平方a sin²A+sin²B=cos²C cos(a+B)×cos(a-B)=1/3,求cos^2(a)-sin^2(B)的值