How to prove that the optimal point for a quasilinear function lies in its extreme points

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I was reading an article about the robust optimization of the MNL choice model,and in one of its proofs it uses the point that if we're tring to solve the minimun of a quasilinear function ,which is the ratio between two linear functions,then it equals to solve the minimum of this function among all its extreme points.This implies that the optimal point for a quasilinear function lies in its extreme points,but I don't know how to prove this.(Plus,it also says this is a conclusion in the book Convex Optimization by Stephen Boyd an Vadenburghe,but I didn't find)

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Suppose that a minimum is attained in the interior. Then the point at which the minimum is attained can be written as a convex combination of extreme points. The function value at the extreme points is strictly higher than the optimal value (as otherwise such point is also optimal and your procedure would find it). Therefore, there is an upper level set that contains those extreme points but not the optimal point, which contradicts quasiconcavity.