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secondary 4 | A Maths

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Ans is 1/12 but I got 16(pie)r

Area of circle, A = πr²

dA/dr = 2πr

dr/dt = dA/dt x dr/dA

= dA/dt ÷ dA/dr

= 8 ÷ 2πr

Since circumference = 96cm and we know that circumference = 2πr,

dr/dt = 8cm²/s ÷ 96cm = 1/12 cm/s

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Subsequently, we define the area of the circle and then evaluate dA/dr when r = the radius which you obtained earlier.

You are also given that dA/dt.

By the chain rule,

dA/dt = dA/dr * dr/dt

Sub in values of dA/dt and dA/dr.