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The rate of change of area $A$ of circle of a radius $r$ is

\[\begin{array}{1 1}(1)2\pi r&(2)2\pi r\frac{dr}{dt}\\(3)\pi r^{2}\frac{dr}{dt}&(4)\pi\frac{dr}{dt}\end{array}\]

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Let A be the radius of the circle at time 't'
$A= \pi r^2$
$\large\frac{dA}{dt} $$=2 \pi r . \large\frac{dr}{dt}$
Hence 2 is the correct answer.
answered May 16, 2014 by meena.p

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