Finding Pi

[PhilOfPerth]

Or, if I read the problem wrong (as it seems Pete and I have two different interpertations of the problem), you might want:

Formula for the Base of an Isosceles Triangle
If you know the side length and height of an isosceles triangle, you can find the base of the triangle using this formula:

b = SQR(a ^ 2 - h ^ 2), where a is the length of a side and h is the height.

how to find the third side of a triangle: formula for the base of an isosceles triangle

where a is the length of one of the two known, equivalent sides of the isosceles.

But we don't know the height - it depends on two things, the side length and (half of) the base length (or the top angle), and the base length is what we want to find.