The domain is $x^2 + y^2 \le 25$.
We use the method of Lagrange multipliers.
Let $g(x, y) = x^2 + y^2 - 25 = 0$.
Then $\nabla f = \lambda \nabla g$, which gives
$(1, 2) = \lambda (2x, 2y)$.
This implies $1 = 2\lambda x$ and $2 = 2\lambda y$.
Thus $x =
Show more…