1. The problem statement, all variables and given/known data Let φ=φ(r) and r=√x2+y2+z2. Find ∂2φ/∂x2. Show that it can be written as (1/r + x2/r3)∂φ/∂r + x2/r2∂2φ/∂r2. 2. Relevant equations Use the identity ∂r/∂x = x/r. 3. The attempt at a solution I think I know ∂φ/∂x. Using the chain rule, it's ∂r/∂x ∂φ/∂r. That gives x/r ∂φ/∂r. If that's wrong it might be because I know you have to take account of all dependences, but I don't actually know how to. So assuming that's ok, I then need to use the product rule, and that gave me 1/r ∂φ/∂r + ∂φ/∂x ∂φ/∂r ∂r/∂x. Which I know is wrong, because it's a show that question!