SOLUTION: Please help me figure out the side length of an equilateral triangle inscribed in a circle with the radius of 21

Question 759127: Please help me figure out the side length of an equilateral triangle inscribed in a circle with the radius of 21
Answer by Cromlix(4381) (Show Source): You can put this solution on YOUR website!
With the equilateral inscribed in the circle
draw one of your radii from a vertex to the centre.
Then take another radii from another vertex to the centre.
You now have a triangle with 30 degrees at the points
touching the vertices and 120 degrees at the centre.
Mark the triangle with A at the centre and B and C
at the vertices.
Use Cosine Rule.
a^2 = b^2 + c^2 - 2bc CosA
a^2 = 21^2 + 21^2 - 2*21*21*Cos 120 degs
a^2 = 1323
a = square root of 1323
a = 36.4 units.
Hope this helps.
:-)
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