The radius of a circle is increasing at the rat... - JAMB Mathematics 2013 Question
The radius of a circle is increasing at the rate of 0.02cms-1. Find the rate at which the area is increasing when the radius of the circle is 7cm.
A
0.75cm2S-1
B
0.53cm2S-1
C
0.35cm2S-1
D
0.88cm2S-1
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Correct Option: D
A = (\pi)r2, (\frac{\delta A}{\delta r}) = 2πr
So, using (\frac{\delta A}{\delta t}) = (\frac {\delta A}{\delta r}) x (\frac {\delta A}{\delta t})
= 2(\pi)r x 0.02
= 2(\pi) x 7 x 0.02
= 2 x (\frac{22}{7}) x 0.02
= 0.88cm2s-1
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