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Question

If the ratio of the radii of two spheres is 2 : 3, then the ratio of their surface areas will be:

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
4 : 9

Sphere Surface Area Ratio Calculation

The surface area of a sphere is calculated using the formula $A = 4\pi r^2$, where $r$ is the radius.

If we have two spheres with radii $r_1$ and $r_2$, their surface areas $A_1$ and $A_2$ are:

  • $A_1 = 4\pi r_1^2$
  • $A_2 = 4\pi r_2^2$

Calculating the Ratio

We are given the ratio of the radii, $r_1 : r_2 = 2 : 3$.

To find the ratio of their surface areas ($A_1 : A_2$), we divide the two formulas:

$ \frac{A_1}{A_2} = \frac{4\pi r_1^2}{4\pi r_2^2} $

The $4\pi$ terms cancel out, leaving:

$ \frac{A_1}{A_2} = \frac{r_1^2}{r_2^2} = \left(\frac{r_1}{r_2}\right)^2 $

Final Ratio Determination

Substitute the given ratio of the radii ($r_1 : r_2 = 2 : 3$):

$ \frac{A_1}{A_2} = \left(\frac{2}{3}\right)^2 = \frac{2^2}{3^2} = \frac{4}{9} $

Therefore, the ratio of their surface areas is 4 : 9.

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