Question

The radius of a circle is increased by $2$ cm from $5$ cm to $7$ cm. Find the increase in area?

Answer 1

Let $r_1=5$cm, $r_2=7$cm

$\Delta r=r_2-r_1=7-5=2$cm

Area of a circle$A=\pi r^2$

$dA=2\pi r.dr=2\pi \times 5\times 2=10\pi {cm}^2$

New area$=A+\Delta A=\pi r^2+dA$

$=\pi \times 5^2+20\pi =45\pi$sq.units