(sina+sinb)^2+(cosa+cosb)^2=?A.(COS(a-b)/2)^2B.4(COS(a-b)/2)^2C..(sin(a-b)/2)^2

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(sina+sinb)^2+(cosa+cosb)^2=?A.(COS(a-b)/2)^2B.4(COS(a-b)/2)^2C..(sin(a-b)/2)^2
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(sina+sinb)^2+(cosa+cosb)^2=?A.(COS(a-b)/2)^2B.4(COS(a-b)/2)^2C..(sin(a-b)/2)^2
(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

(sina+sinb)^2+(cosa+cosb)^2=?A.(COS(a-b)/2)^2B.4(COS(a-b)/2)^2C..(sin(a-b)/2)^2
(sina+sinb)^2+(cosa+cosb)^2
=sin²a+2sina*sinb+sin²b+cos²a+2cosa*cosb+cos²b
=2+2(sina*sinb+cosa*cosb)
=2+2cos(a-b)

展开=1+1+2（sinasinb+cosacosb)=2+2cos(a-b)

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