cos2a=2/3 sin^4 a+cos^4a=
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cos2a=2/3sin^4a+cos^4a=cos2a=2/3sin^4a+cos^4a=cos2a=2/3sin^4a+cos^4a=sin²2a+cos²2a=1所以sin&
cos2a=2/3 sin^4 a+cos^4a=
cos2a=2/3 sin^4 a+cos^4a=
cos2a=2/3 sin^4 a+cos^4a=
sin²2a+cos²2a=1
所以sin²2a=5/9
(2sinacosa)²=5/9
所以sin²acos²a=5/36
原式=(sin²a+cos²a)²-2sin²acos²a
=1²-2×5/36
=13/18