(sina+sinb)^2+(cosa+cosb)^2=?A.(COS(a-b)/2)^2B.4(COS(a-b)/2)^2C..(sin(a-b)/2)^2
问题描述：
(sina+sinb)^2+(cosa+cosb)^2=?
A.(COS(a-b)/2)^2
B.4(COS(a-b)/2)^2
C..(sin(a-b)/2)^2
答
展开=1+1+2（sinasinb+cosacosb)=2+2cos(a-b)
答
(sina+sinb)^2+(cosa+cosb)^2
=sin²a+2sina*sinb+sin²b+cos²a+2cosa*cosb+cos²b
=2+2(sina*sinb+cosa*cosb)
=2+2cos(a-b)