Question

If the sum and the difference of the radii of two concentric circles are 7 cm and 3 cm respectively, then find the area in between those circles.

• A. 11 $c{m}^2$
• B. 33 $c{m}^2$
• C. 66 $c{m}^2$
• D. 132 $c{m}^2$

Answer 1

The correct option is C 66 $c{m}^2$

Given : R+r = 7 cm, R - r = 3 cm ...where r, R are inner and outer radii respectively.
area of the ring = $\pi$($R^2-r^2$)
= $\pi$(R+r)$\times$(R-r)
= $\frac{22}{7}$$\times$7$\times$3
= 22$\times$3 = 66 $c{m}^2$