Question

The area of a ring is $40\pi {\text{cm}}^2$. If the width of the ring is $4$ cm, then the sum of the radii of circles

• A. $4$
• B. $5$
• C. $3$
• D. $10$

Answer 1

Answer: (D)

$10$

Given width of ring is $4$.

Hence, $R_1-R_2=4cm$.........[1]

Area of ring$=\pi (R_1^2-R_2^2)$

$⟹40\pi =\pi (R_1-R_2)(R_1+R_2)$

$⟹40=4(R_1+R_2)$........................[from $1$]

Hence, $R_1+R_2=10cm$