sin(a+b)=4/5cos(a-b)=12/13求cos 2a=?

sin(a-b)*cos a-cos(a-b)*sin a=1/5,则cos 2b的值是 当a=3π/4时,sin(a+b)+cos(a+b)+sin(a-b)+cos(a-b) 由cos(a+b)=cos a cos b-sin a sin b cos(a-b)=cos a cos b+sin a sin b解题设a为锐角,证：1、2分之根3乘cos a + 2分之1乘sin a=cos(6分之π-a)2、cos a-sin a=根号2cos(4分之π+a) Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2 sin(a)*cos(a)=4,那么sin(a) cos(b)=? 如何证明sin(a+b)=sin(a)cos(b)+cos(α)sin(b) A,B为锐角,sin A=4/5,COS(A+B)=5/13,求sin B 已知a,b都是锐角,sin a =4/5,cos(a+b),求sin b 的值,速求! sin(a+b)cosa-cos(a+b)sina=5/13 则sin（π-B)=? cos(a-b)cos(b-c)+sin(a-b)sin(b-c)= 已知 sin a +sin b =1/4cos a +cos b =1/3求 tan (a+b) cos^2a-cos^2b=m,那么sin(a+b)sin(a-b)=? cos²a-cos²b=c,则sin(a+b)sin(a-b)= cos∧a-cos∧b=t则sin(a+b)sin(a-b)= 证明sin(A+B)sin(A-B)=cos^2B-cos^2A cos(a+b)cos(a-b)+sin(a+b)sin(a-b)=-3/5,求sin(3π/2+2b) 若cos(a+B)*cosa+sin(a+B)*sina=-4/5,则sin(π/2-B)我是白痴 已知sin[a-b]cos a-cos[b-a]sin a=3/5,b是第三象限角,求sin[b+5π/4]的值第一题1/2cos x-√3/2sin x第二题√3sin x+cos x第三题√2[sin x-cos x]第四题√2cos x-√6sin x