cos^2(a+b)=?
求证:a^2(cos^2b-cos^2c)+b^2(cos^c-cos^2a)+c^2(cos^2a-cos^2b)=0
Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2
cos(a+B)×cos(a-B)=1/3,求cos^2(a)-sin^2(B)的值
若sin^4a/sin^2b+cos^4a/cos^2b=1,证明sin^4b/sin^2a+cos^4b/cos^2a
由cos(a+b)=cos a cos b-sin a sin b cos(a-b)=cos a cos b+sin a
求证:cosa+cosb=2cos(a+b)/2cos(a-b)/2 ,
已知cos(a-b)=-12/13 cos(a+b)=12/13且π/2
已知cos(a+b)cos(a-b)=1/3 ,求(cosa)^2-(cosb)^2
cos(A+B)cos(A-B)=1/4 求cosA^2+cosB^2
化简2cos(a+b)cosa-cos(2a+b)=
在三角形ABC中,证明2sinA*sinB=-[cos(A+B)-cos(A-B)]
(sinA+sinB)/(cos A +cos B) =tan (A+B)/2 怎么证明?