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65. show that of all the isosceles triangles with a given perimeter, the one with the greatest area is equilateral.

User Paddyd
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1 Answer

11 votes

Answer:

Below.

Explanation:

I can do this using calculus but the algebra will be a bit messy.

Let the 2 equal sides of isosceles triangle be x and the base be y units long.

Then perimeter p = 2x + y.

Area A = 0.5yh where h is the altitude of the triangle.

h = sqrt [x^2 - (0.5y)^2]

= [x^2 - (0.5y)^2]^0.5

So A = 0.5y * [x^2 - (0.5y)^2]^0.5

From the perimeter formula y = p - 2x so:

A = 0.5(p-2x) * [x^2 - (0.5(p-2x)^2]^0.5

A = 0.5(p-2x) * [x^2 - 0.25(p^2 - 4xp + 4x^2)]^0.5

A = 0.5(p-2x) * [x^2 - 0.25p^2 + xp - x^2]^0.5

A = (0.5p-x) * [xp - 0.25p^2]^0.5

We now find the derivative and equate it to zero to find the value of x which corresponds to a maximum area.

We use the product rule:

dA/dx = (0.5p-x)* 0.5(xp - 0.25p^2)^(-0.5) * p - 1(xp - 0.25p^2)^0.5 = 0

p(0.5p - x) / (2((xp - 0.25p^2)^0.5 - (xp - 0.25p^2)^0.5 = 0

p(0.5p - x) / (2((xp - 0.25p^2)^0.5 = (xp - 0.25p^2)^0.5

Now multiplying though by (2((xp - 0.25p^2)^0.5 :-

p(0.5p - x) = 2 (xp - 0.25p^2)

0.5p^2 - px = 2px - 0.5p^2

p^2 = 3px

x = p^2/3p = p/3

So for a maximum value of the area x = p/3.

But y = p - 2x

= p - 2(p/3)

= p - 2/3p

= p/3

- but we've just shown that x = p/3

That means that x = y and the triangle is Equilateral.

User Lamak
by
8.6k points

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