y=(1+x^2)^2=1+2x^2+x^4;y'=0+2*2x+4x^3=4x^3+4x.dy=2(1+x^2)*d(1+x^2)=2(1+x^2)*(0+2x)dx=(4x^3+4x)dx;dy/dx=4x^3+4x
