Consider the circle of radius r pictured below with central angle , measured in radians, and subtended arc of length s. Prove that the area of the shaded sector is
.
Let's say the entire circle has area
. Using proportionality arguments, the area of a portion of a circle is then
which reduces to
.
Is that right? It seems too easy for a prove. I got this question from a online book which doesn't contain the answer, so I can only resort to stack exchange.