(sinα+cosα)^2=sin^2α+cos^2α+2*sinα*cosα=a^2=1+2*sinα*cosα
所以,sinα*cosα=（a^2-1）/2
(sin^2α+cos^2α)^2=sin^4α+cos^4α+2*sin^2α*cos^2α
所以,sin^4α+cos^4α=1-2*sin^2α*cos^2α=1-（(a^2-1)/2）^2*2