y=x^2-t^2
t--->0时,y--->x^2
t--->x时,y--->0
F(x)= ∫下限0上限x [t f(x^2-t^2)] dt
= ∫下限0上限x [-1/2 f(x^2-t^2)] d(x^2-t^2)
=1/2* ∫下限0上限x ^2[f(y)] dy
F'(x)=1/2*f(x^2)*2x=xf(x^2)