Area of inner circle with radius r = πr2
Radius of outer circle = r + h
Area of outer circle = π(r + h)2
Area of circular path with width = h
= π(r + h)2 – πr2
By using (a + b)2 = a2 + b2 +2ab
= π(r2 + h2 + 2rh) – πr2
= πr2 + πh2 + 2πrh – πr2
= πh(2r + h)… Proved
