Question
Question: A stone is dropped into a quiet lake and waves move in a circle at a speed of 4 cm/s. At the instant...
A stone is dropped into a quiet lake and waves move in a circle at a speed of 4 cm/s. At the instant when the radius of the circular wave is 10 cm, the enclosed area increases at the rate :
A
100p cm2/s
B
80p cm/s
C
40 cm2/s
D
None of these
Answer
None of these
Explanation
Solution
dtdr = 4cm/sec
when r = 4cm, A = pr2
dtdA = p.2rdtdr = p . 2 × 4 × 4 = 32p cm2/sec