Geometry · real student question

The similarity ratio of two similar polygons is 2 to 3. Comparing the smaller polygon to the larger polygon, find the ratio of their areas. Choose from 2:3, 3:2, 9:4, or 4:9.

Question

The similarity ratio of two similar polygons is 2:32:3. Comparing the smaller polygon to the larger polygon, find the ratio of their areas.

2:33:29:44:92:3 \qquad 3:2 \qquad 9:4 \qquad 4:9

Step-by-step solution

  1. Separate the two different ratios in play. A similarity ratio is a ratio of lengths. The question asks for a ratio of areas, and those two ratios are never equal unless the figures are congruent. Picking 2:32:3 straight off the page is the standard trap here.

  2. See where the square comes from. Split each polygon into triangles. Every triangle in the larger figure has base and height both 32\tfrac{3}{2} times the matching triangle in the smaller one, so its area 12bh\tfrac12 bh is multiplied twice:

    arealargeareasmall=12(32b)(32h)12bh=(32)2\frac{\text{area}_{\text{large}}}{\text{area}_{\text{small}}}=\frac{\tfrac12\left(\tfrac32 b\right)\left(\tfrac32 h\right)}{\tfrac12 bh}=\left(\frac{3}{2}\right)^{2}

    Adding the triangles back up keeps that same factor, so the rule holds for the whole polygon.

  3. Apply the rule in the order the question asks. Smaller to larger means the side ratio is 23\dfrac{2}{3}, so square it:

    AsmallAlarge=(23)2=2232=49\frac{A_{\text{small}}}{A_{\text{large}}}=\left(\frac{2}{3}\right)^{2}=\frac{2^2}{3^2}=\frac{4}{9}

  4. Check the answer against the distractors. 2:32:3 forgets to square, 3:23:2 and 9:49:4 both reverse the order and describe the larger figure compared to the smaller. Only 4:94:9 is both squared and in the right order.

  5. State the result. The ratio of the areas, smaller to larger, is

    Asmall:Alarge=4:9A_{\text{small}}:A_{\text{large}}=4:9

Answer

Asmall:Alarge=22:32=4:9A_{\text{small}}:A_{\text{large}} = 2^2:3^2 = 4:9

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