Prove that (grad)^2 f(r) = D^2/dr^2 + (2/r) Df/Dr
Where D^2/dr^2 refers to the second partial derivative of f , w.r.t. r and
Df/ Dr refers to partial derivative of f, w.r.t r
vector r = xi + yj + zk and f(r) is twice differentiable
How do we start with f(r) and finish with f?
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