<Dd> <Dl> <Dd> ∫ − r r r 2 − x 2 d x (\ displaystyle \ int _ (- r) ^ (r) (\ sqrt (r ^ (2) - x ^ (2))) \, dx) </Dd> </Dl> </Dd> <Dl> <Dd> ∫ − r r r 2 − x 2 d x (\ displaystyle \ int _ (- r) ^ (r) (\ sqrt (r ^ (2) - x ^ (2))) \, dx) </Dd> </Dl> <Dd> ∫ − r r r 2 − x 2 d x (\ displaystyle \ int _ (- r) ^ (r) (\ sqrt (r ^ (2) - x ^ (2))) \, dx) </Dd> <Dl> <Dd> = ∫ − π 2 π 2 r 2 (1 − sin 2 ⁡ θ) ⋅ r cos ⁡ θ d θ (\ displaystyle = \ int _ (- (\ frac (\ pi) (2))) ^ (\ frac (\ pi) (2)) (\ sqrt (r ^ (2) (1 - \ sin ^ (2) \ theta))) \ cdot r \ cos \ theta \, d \ theta) </Dd> </Dl>

Why pi r2 is the area of a circle