Hint to Riddle #76: Area of an Annulus

76. The diagram shows two concentric annuli (two dimensional ring shapes,) given that the two red lines are the same length, prove that the shaded areas are the same too.
two concentric annuli with equal chord length All you need to prove the answer is some really basic geometry. The area of a circle is πr2 and Pythagoras theorem.
diagram for calculating the area of an annulus
That's really all there is to it.

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