Answer:
Let r be the radius of circle at any time t. Then, \[\frac{dr}{dt}=0\cdot 7\,\,\text{cm/s}\] Now, let C denotes the circumference of circle. Then, \[C=2\pi r\] \[\Rightarrow \] \[\frac{dC}{dt}=2\pi \frac{dr}{dt}\] \[\Rightarrow \] \[\frac{dC}{dt}=2\pi (0.7)=1.4\pi \,cm/s\] Hence, the rate of increase of its circumference is\[1.4\pi \,cm/s\],
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