Q40 / 90 · JEE Main 2014 (19 Apr Online) · Mathematics

If the volume of a spherical ball is increasing at the rate of 4π cc/sec then the rate of increase of its radius in cm/sec, when the volume is 288π cc is 

  • A
    1 9
  • B
    1 6
  • C
    1 2 4
  • Correct answer
    D
    1 3 6

Solution

Volume of sphere of radius r is V=43πr3

Given V=288π cc

⇒288π=43πr3

⇒r3=288×34=216

⇒r=6 cm.

Again, V=43πr3

Differentiate w.r.t. 't'

⇒dVdt=4πr2drdt

Put the given values, to get

⇒4π=4π×62×drdt

⇒drdt=136 cm/sec.

Hence, the correct answer is option: 'D'.

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