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单词 BoundOnAreaOfRightTriangle
释义

bound on area of right triangle


We may bound the area of a right triangleMathworldPlanetmath in terms of its perimeterPlanetmathPlanetmath.The derivation of this bound is a good exercise in constrainedoptimization using Lagrange multipliers.

Theorem 1.

If a right triangle has perimeter P, then its area is bounded as

A3-224P2

with equality when one has an isosceles right triangle.

Proof.

Suppose a triangle has legs of length x and y. Then its hypotenuseMathworldPlanetmathhas length x2+y2, so the perimeter is given as

P=x+y+x2+y2.

The area, of course, is

A=12xy.

We want to maximize A subject to the constraint that P be constant.This means that the gradient of A will be proportional to the gradientof P. That is to say, for some constant λ, we will have

Ax=λPx
Ay=λPy

Together with the constraint, these form a system of three equationsfor the three quantities x, y, and λ. Writing them outexplicitly,

12y=λ(1+xx2+y2)
12x=λ(1+yx2+y2)
P=x+y+x2+y2

Not that we cannot have λ=0 because that would mean that allsides of our triangle would have zero length. Hence, we may eliminateλ between the first two equations to obtain

x(1+xx2+y2)=y(1+yx2+y2),

which may be manipulated to yield

(x-y)(1+x+yx2+y2)=0.

We have two case to consider — either the first factor or the secondfactor may equal zero. If the second factor equals zero,

1+x+yx2+y2=0,

move the “1” to the other side of the equation and cross-multiplyto obtain

x+y=-x2+y2.

Since we want x0 and y0 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

x-y=0.

In this case, x equals y. Imposing this condition on the constraint,we see that

P=(2+2)x,

so we have the solution

x=P2+2=2-22P
y=P2+2=2-22P.

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更新时间:2025/5/5 0:59:45