1,sinα+cosα=√2
两边平方得1+2sinαcosα=2
sinαcosα=1/2
sin^4α+ cos^4α=(sin^2α+ cos^2α)^2-2sin^2α cos^2α=1-2*1/4=1/2
2.sinα+cosα=1/5 (1)
两边平方得1+2sinαcosα=1/25
2sinαcosα=-24/25
(sinα-cosα)^2=1-2sinαcosα=49/25
sinα-cosα=7/5(2)sinα-cosα=-7/5(舍去)(因为0