bound on area of right triangle
We may bound the area of a right triangle in terms of its perimeter
.The derivation of this bound is a good exercise in constrainedoptimization using Lagrange multipliers.
Theorem 1.
If a right triangle has perimeter , then its area is bounded as
with equality when one has an isosceles right triangle.
Proof.
Suppose a triangle has legs of length and . Then its hypotenusehas length , so the perimeter is given as
The area, of course, is
We want to maximize subject to the constraint that be constant.This means that the gradient of will be proportional to the gradientof . That is to say, for some constant , we will have
Together with the constraint, these form a system of three equationsfor the three quantities , , and . Writing them outexplicitly,
Not that we cannot have because that would mean that allsides of our triangle would have zero length. Hence, we may eliminate between the first two equations to obtain
which may be manipulated to yield
We have two case to consider — either the first factor or the secondfactor may equal zero. If the second factor equals zero,
move the “1” to the other side of the equation and cross-multiplyto obtain
Since we want and but the right-hand side isnon-positive, the only option would be to have a trianagle ofzero area.The other possibility was to have the second factor equal zero,which would give
In this case, equals . Imposing this condition on the constraint,we see that
so we have the solution
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