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'If f is a twice differentiable function of a single variable, find f = z(sqrt(x^2+y^2)) that satisfies d^2z/dx^2 +d^2z/dy^2 = x^2 +y^2

(ALL d's ARE MEANT TO BE PARTIAL DERIVATIVES)

i know dz/dx=(dz/du).(du/dx)

i can find du/dx but i don't know how to find dz/du