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Property of Similar Triangles

The Property of Similar Triangles specializes the definition of similar figures to triangles. If ABC\triangle ABC is similar to XYZ\triangle XYZ, with each side labeled by the lowercase letter matching its opposite vertex (side aa opposite AA, side bb opposite BB, side cc opposite CC; side xx opposite XX, side yy opposite YY, side zz opposite ZZ), then two conditions hold:

  1. All corresponding angle measures are equal: mA=mXm\angle A = m\angle X, mB=mYm\angle B = m\angle Y, and mC=mZm\angle C = m\angle Z.
  2. All corresponding side lengths form equal ratios: ax=by=cz\frac{a}{x} = \frac{b}{y} = \frac{c}{z}

Because every pair of corresponding sides shares the same ratio, knowing three sides of one triangle and one side of the other is sufficient to determine the remaining unknown sides by setting up and solving a proportion.

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Updated 2026-05-01

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