I have an equation: r^2 = x^2 So I found out dr/dx = x/r. But when I try to find the second derivative, I get d2r/dx2 = -x^2/r^3 when the text says it should be (r^2 - x^2)/r^3. Can anyone help? My working out: r^2 - x^2 = 0 r^2 = x^2. Assume r is a function of x. rr' = x (first derivative found correctly) rr'' + r'(x/r) = 1 (apply chain rule and sub in answer for first derivative) rr'' + x^2/r^2 = 1 (sub in first derivative) So where have I gone wrong?