What is the largest area of an isoceles triangle with

• May 5th 2009, 08:24 AM
gtgamer140@yahoo.com
What is the largest area of an isoceles triangle with
What is the largest area of an isosceles triangle with the perimeter 1 cm?
• May 5th 2009, 09:00 AM
Infophile
Hello,

I find $\color{red}\frac{\sqrt{3}}{36}$ cm.

I give my solution after diner.

:)
• May 5th 2009, 09:12 AM
running-gag
Quote:

Originally Posted by gtgamer140@yahoo.com
What is the largest area of an isosceles triangle with the perimeter 1 cm?

Hi

Let a be the equal length of 2 sides and b the length of the 3rd side.

The perimeter is p = 1 = 2a+b

The are is given by the formula $S = \sqrt{\frac{p}{2}\left(\frac{p}{2}-a\right)\left(\frac{p}{2}-a\right)\left(\frac{p}{2}-b\right)}$

$S = \sqrt{\frac{1}{2}\left(\frac{1}{2}-\frac{1-b}{2}\right)\left(\frac{1}{2}-\frac{1-b}{2}\right)\left(\frac{1}{2}-b\right)}$

$S = \sqrt{\frac{1}{2}\:\frac{b}{2}\:\frac{b}{2}\:\frac {1-2b}{2}}$

$S = \frac{b}{4}\:\sqrt{1-2b}$

Set the derivative of S to 0 to find b_max then S_max
• May 5th 2009, 09:29 AM
Infophile
Exactly,

But it's not necessary to know this formula.

You have $h^2=a^2-\left(\frac{b}{2}\right)^2$ with $b=1-2a$ and $S=\frac{bh}{2}$.

:)