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 cos(a+b)=cos(a)*cos(b)-sin(a)*sin(b) 用三角形证明 若sin^4a/sin^2b+cos^4a/cos^2b=1,证明sin^4b/sin^2a+cos^4b/cos^2a 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 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 求证:cos(a+b)cos(a-b)=cos平方b-sin平方a 化简 sin^4a-cos^4b sin(a)*cos(a)=4,那么sin(a) cos(b)=? cosb=cos[(a+b)-a]=cos(a+b)cosa+sin(a+b)sina sin²(a+b)+cos²(a+b)= 公式：sin(a+b)=?...sin(a-b)=?...cos(a+b)=?.cos(a-b)=?...