sin^4+cos^4=5/9,是不是这样sin^4α+cos^4α=5/9?sin^4α+cos^4α=(sin²α+cos²α)²-2sin²αcos²α=1-1/2*(sin2α)²=5/91/2*(sin2α)²=4/9(sin2α)²=8/9α是第三象限的角π...