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JEE Mains · Maths · STD 12 - 6. Application of derivatives

Water is being filled at the rate of \(1\, cm ^{3} / sec\) in a right circular conical vessel (vertex downwards) of height \(35\, cm\) and diameter \(14 \,cm\). When the height of the water level is \(10\, cm\), the rate (in \(cm ^{2} / sec\) ) at which the wet conical surface area of the vessel increases is

  1. A \(5\)
  2. B \(\frac{\sqrt{21}}{5}\)
  3. C \(\frac{\sqrt{26}}{5}\)
  4. D \(\frac{\sqrt{26}}{10}\)
Verified Solution

Answer & Solution

Correct Answer

(C) \(\frac{\sqrt{26}}{5}\)

Step-by-step Solution

Detailed explanation

Given \(\frac{ dV }{ dt }=1 \Rightarrow \frac{ d }{ dt }\left(\frac{\pi r ^{2} h}{3}\right)=1\) \(\Rightarrow \frac{ d }{ dt }\left(\frac{5 \pi}{3} r ^{3}\right)=1 \Rightarrow r ^{2} \frac{ dr }{ dt }=\frac{1}{5 \pi}\) Let wet conical surface area \(=S\)…