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The ratio of the radius of two circles is 1: 3, and then the ratio of their areas is?

  • A.1:3
  • B.1:6
  • C.1:9
  • D.1:5

Answer: C

Area varies as the square of the radius, so the ratio is squared.

\[ \frac{A_{1}}{A_{2}} = \frac{\pi r_{1}^{2}}{\pi r_{2}^{2}} = \left(\frac{1}{3}\right)^{2} = \frac{1}{9} \]

So the areas are in the ratio \( 1 : 9 \). Carrying the linear ratio straight through gives \( 1 : 3 \) - the circumferences, not the areas.

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