To find the area of a triangle you need the formula (base*height)/2. We know the base is 25in however we need to find the height to have everything for the formula. If you draw a picture of this equilateral triangle so the bottom is a flat side, the height goes straight up the middle, splitting the triangle into two equal pieces. However, to find the height, you have to use Pythagoras's theorem (a^2+b^2=c^2). c is the hypotenuse so that is the 25in, a is half the width of the base, 12.5in, and b is what you are looking for. When solved, b becomes about 21.65, or if you have to be exact- sqrt{468.75}. Then plug this in for the height in the original equation.
(25*sqrt{468.75})/2≈270.63 in^2
