cos2a=2/3 sin^4 a+cos^4a=

问题描述:

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