We can use the method of Lagrange multipliers to find the maximum and minimum of $f$ subject to the constraint. The Lagrange function is $L(x, y, z, \lambda)=f(x, y, z)-\lambda(g(x, y, z)-c)$, where $g(x, y, z)=x^{2}+y^{2}+z^{2}$ and $c=25$.
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