y'=1/(x+根号下x^2+1)*(x+根号下x^2+1)'=1/(x+根号下x^2+1)*(1+x/根号下x^2+1)=1/(x+根号下x^2+1)*(根号下x^2+1+x)/根号下x^2+1=1/根号下(x^2+1)