alleryRate of change of area of a circle (A = πr²) with respect to radius:
A2πr (= circumference)
Br²
Cπr
Dπ
Answer & Solution
Correct answer: A. 2πr (= circumference)
dA/dr = 2πr.
Interestingly, this equals the circumference — geometrically because thin shell of radius r and width dr has area 2πr × dr.
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