y=(sin(x/4))^4+(cos(x/4))^4
==(sin(x/4))^4+(cos(x/4))^4 +2(sin(x/4))^2(cos(x/4))^2-2(sin(x/4))^2(cos(x/4))^2
=1 - (sin(x/2))^2/2
=1-(1-cosx)/2/2
=3/4+cosx/4
y'=-sinx/4
在求导前应该考虑一下对三角形的化简.