The length of radius of a spherical gas ballon increases from 7 cm to 21 cm as air is being pumped into it, then find the ratio of surface areas of the ballon in two cases .

Explanation

The previous radius of the balloon `=4xx(22)/(7)xx7^(2)` sq.cm
`therefore` the ratio of the whole surface areas of the balloon in two cases
`=4xx(22)/(7)xx7^(2):4xx(22)/(7)xx21^(2)=(7xx7)/(21xx21)=(1)/(9)=1:9`.
Hence the required ratio=1:9.