y=1/(1+x^2) *e^arctanx
y'=(1/(1+x^2))'e^arctanx+1/(1+x^2)(e^arctanx)'
=1/(1+x^2)^2*(1+x^2)'*e^arctanx+1/(1+x^2)*e^arctanx*(arctanx)'
=2xe^arctanx /(1+x^2)^2 +e^arctanx/(1+x^2) *1/(1+x^2)
=(2x+1)e^arctanx/(1+x^2)^2