NCERT Solutions for Class 9 Maths Chapter 13
Important NCERT Questions
Surface Areas and Volumes
NCERT Books for Session 2022-2023
CBSE Board, UP Board and Other state Boards
EXERCISE 13.4
Page No:225
Questions No:4
The radius of a spherical balloon increases from 7 cm to 14 cm as air is being pumped into it. Find the ratio of surface areas of the balloon in the two cases.
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First case:
Radius of balloon r = 7 cm
Surface area of balloon = 4πr²
= 4 × 22/7 × 7 × 7 = 4 × 22 × 7 = 616 cm²
Hence, the surface area of ballon is 616 cm²
Second case:
Radius of balloon R = 14 cm
Surface area of bolloon = 4πR²
= 4 × 22/7 × 14 × 14
= 4 × 22 × 2 × 14
= 2464 cm²
Hence, the surface area of balloon is 2464 cm².
Ratio of surface area = 616/2464 = 1/4
Hence, the ratio of surface areas in two cases is 1:4