1.

The rate of change of surface area of a sphere of radius r when the radius is increasing at the rate of 2 cm/sec is proportional toA. `(1)/(r^(2))`B. `(1)/(r )`C. `r^(2)`D. r

Answer» Correct Answer - D
Let S be the surface area and r be the radius of the sphere at any time t. Then,
`S=4pi r^(2)`
`implies (dS)/(dt)=8pi r(dr)/(dt)`
`implies (dS)/(dt)=8pirxx2" "[(dr)/(dt)=2cm//sec]`
`implies (dS)/(dt)=16pir implies (dS)/(dt)propr`


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