1.

If the circumference of the circle is changing at the rate of 5 cm/s then what will be rate of change of area of the circle if the radius is 6cm.(a) 20 cm^2/s(b) 40 cm^2/s(c) 70 cm^2/s(d) 30 cm^2/sThe question was posed to me in an international level competition.The origin of the question is Derivatives Application in division Application of Derivatives of Mathematics – Class 12

Answer»

Right choice is (d) 30 cm^2/s

Explanation: The circumference of the CIRCLE is given by C=2πr, where r is the radius of the circle.

∴\(\FRAC{dC}{dt}\)=2π.\(\frac{DR}{dt}\)=5 cm/s

\(\frac{dr}{dt}\)=5/2π cm/s

\(\frac{dA}{dt}\)|r=6=2πr.\(\frac{dr}{dt}\)=2πr.\(\frac{5}{2 \pi}\)=5r=5(6)=30 cm^2/s.



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