左边=
sin^22a+cos^22a+sin^22b+cos^22b+2cos2acos2b-2sin2asin2b
=2+2cos(2a+2b)
=2+2[2cos^2(a+b)-1]
=4cos^2(a+b)
=右边
证明(sin2a-sin2b)^2+(cos2a+cos2b)^2=4*(cos(a+b))^2
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