The volume of one sphere is 27 times that of another sphere. Calculate the ratio of their: (i) radii (ii) surface areas
The volume of one sphere is 27 times that of another sphere. Calculate the ratio of their: (i) radii (ii) surface areas

The volume of first sphere = 27 x volume of second sphere

Let the radius of the first sphere = r1 and, radius of second sphere = r2

(i) Then, according to the question we have

    \[\begin{array}{*{35}{l}} 4/3\text{ }\pi {{r}_{1}}^{3}~=\text{ }27\text{ }\left( 4/3\text{ }\pi {{r}_{2}}^{3} \right)  \\ {{r}_{1}}^{3}/\text{ }{{r}_{2}}^{3}~=\text{ }27  \\ {{r}_{1}}/\text{ }{{r}_{2}}~=\text{ }3/\text{ }1  \\ Thus,\text{ }{{r}_{1}}:\text{ }{{r}_{2~}}=\text{ }3:\text{ }1  \\ \end{array}\]

(ii) Surface area of the first sphere = 4 πr12

And the surface area of second sphere = 4 πr22

Ratio of their surface areas

    \[=\text{ }4\text{ }\pi {{r}_{1}}^{2}/\text{ }4\text{ }\pi {{r}_{2}}^{2}~=\text{ }{{r}_{1}}^{2}/\text{ }{{r}_{2}}^{2}~=\text{ }{{3}^{2}}/\text{ }{{1}^{2}}~=\text{ }9\]

Hence, the ratio = 9: 1