Question

The rate of change of area of a circle with respect to its radius at r = 2 cms is
1. 4 π
2. 2
3. 2 π
4. 4
5. π

Answer 1

Correct Answer - Option 1 : 4 π

Concept:

Rate of change of 'x' is given by \(\rm \frac {dx}{dt}\)

Calculation:

Given r = 2cms

Area of a circle A = π r²

\(\rm {dA \over dr}\) = 2πr

At r = 2cms

\(\rm {dA \over dr}\) = 2π × 2

\(\rm {dA \over dr}\) = 4π