A stone is dropped into a pond. Waves in the form of circles are generated and radius of outermost ripple increases at the rate of 5 cm/sec. then area increased after 2 sec is . . .

Options

A. `100 pi cm^(2)//sec`
B. `40 cm^(2)//sec`
C. `50 cm^(2)//sec`
D. `25 cm^(2)//sec`

Answer

A

Explanation

We have, `(dr)/(dt)=5 cm//sec`
`A=pir^(2)rArr(dA)/(dt)=2pir(dr)/(dt)`
`rArr(dA)/(dt)=2pi(10)/(5)=100 picm^(2)//sec`