Similarity Criteria Math Example 3

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Example 3

medium
Triangle ABC has sides 55, 1212, 1313. Triangle DEF has sides 1010, 2424, 2626. Prove the triangles are similar and state the criterion used.

Solution

  1. 1
    Step 1: Order the sides of each triangle from smallest to largest. ABC\triangle ABC: 5,12,135, 12, 13. DEF\triangle DEF: 10,24,2610, 24, 26.
  2. 2
    Step 2: Compute the ratios of corresponding sides: 105=2\dfrac{10}{5} = 2, 2412=2\dfrac{24}{12} = 2, 2613=2\dfrac{26}{13} = 2.
  3. 3
    Step 3: All three ratios are equal (k=2k = 2), so every pair of corresponding sides is proportional.
  4. 4
    Step 4: By SSS~ (Side-Side-Side Similarity), ABCDEF\triangle ABC \sim \triangle DEF with scale factor 22.

Answer

ABCDEF\triangle ABC \sim \triangle DEF by SSS~ with scale factor k=2k = 2.
SSS~ states that if all three pairs of corresponding sides of two triangles are in the same ratio, the triangles are similar. Note that both triangles here are also right triangles (52+122=1325^2 + 12^2 = 13^2 and 102+242=26210^2 + 24^2 = 26^2), so AA~ would also apply since both contain a 90°90° angle.

About Similarity Criteria

Three sets of conditions that guarantee two triangles are similar: AA (two pairs of equal angles), SAS~ (two pairs of proportional sides with equal included angle), and SSS~ (all three pairs of sides in the same ratio).

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