Posted by Katie on Wednesday, June 13, 2012 at 4:21pm.
What is the area of a regular pentagon with a side length of 10 inches and a distance from the center to a vertex of 10 inches?
Round your answer to the nearest tenth.

geometry  Reiny, Wednesday, June 13, 2012 at 5:07pm
Look at one of the five isosceles triangle
each has a 72 degree angle with sides of 10 containing that angle.
Use the cosine law to find the base of that triangle
x^2 = 10^2 + 10^2  2(10)(10)cos 72
x = ...
Once you have that, find the area of the triangle (1/2)(10)(10sin(above angle)
then multiply by 5