# maths

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a circular ink-blot grows at a rate of 2 sq cm per second. find the rate at which the radius is increasing after (28/11)seconds.

• maths -

assuming it had zero area at t=0,

a = pi r^2
da/dt = 2pi r dr/dt
2 = 2pi (28/11) dr/dt
11/(28pi) = dr/dt

• maths -

i tried this way but answer not coming..... :/

ans is 0.25

• maths -

nope. I don't buy it. I think the answer lost a factor of 2 somewhere. 11/(28pi) is 0.125

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