and the purple area ?
If the triangle shown is an equilateral triangle, compare the orange area
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Since the concentric center O of the two circles is also the centroid of the equilateral triangle, if the radius of the orange circle is r , the radius of the big circle is 2 r .
We note that the purple area A p is one-third of the area of the big circle minus the area of the orange circle. Then A p = 3 π ( 2 r ) 2 − π r 2 = π r 2 . Therefore A o = A p .