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Let y=f(x).phi(x) and z=f'(x).phi'(x). p...

Let `y=f(x).phi(x) and z=f'(x).phi'(x).` prove that `1/y*(d^2y)/(dx^2)=1/f*(d^2f)/(dx^2)+1/phi*(d^2phi)/(dx^2)+(2z)/(fphi).`

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