（cosα)^4-(sinα)^4=1/2
[（cosα)^2-(sinα)^2]*[（cosα)^2+(sinα)^2]=1/2
（cosα)^2-(sinα)^2=1/2 1)
（cosα)^2+(sinα)^2=1 2)
1)-2),得
2(sinα)^2=1/2
sinα=1/2