2015 MI P2 Q2
2015 MI P2 Q2
Prelims
[Two circles are concentric when they have the same centre but different radii.]
Two concentric circles have radii \(R\) and \(r\), where \(R>r\) . If \(R\) increases at a constant rate of \(2\) cm/s and the area between the \(r\) circles is constant at \(20\) cm\(^{2}\), find the rate at which r is increasing when \(R=5\) cm.
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Answer:\({\frac{\mathrm{d}r}{\mathrm{d}t} = 2.32\text{ cm/s}\hspace{0.5em} (3\text{ s.f.})}\)
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