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The rate of change of an area of a circle with respect to its radius which is 7 cms is
1. 21π
2. 14π
3. 7π
4. 12π

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Correct Answer - Option 2 : 14π


Rate of change of 'x' with respect to 't' is given by \(\rm \frac {dx}{dt}\)


Given r = 7cms

Area of a circle A = π r2

\(\rm {dA \over dr}\) = 2πr

At r = 7cms

\(\rm {dA \over dr}\) = 2π × 7 

\(\rm {dA \over dr}\) = 14π 

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