Stage 9 - Q10 - 3 marks
19126
A ring has outer radius \(R\) and inner radius \(r\), with \(0<r<R\). Its area is \(A=\pi(R^2-r^2)\). Make r the subject, state the allowed range of A, then find r when \(R=5\) cm and \(A=9\pi\) square cm.
Hint
Subtract the ring area from the full outer disk area.