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Similar triangles have equal angles. The ratio of length of corresponding sides is equal. a' / a = b' / b = c' / c = k. k - the ratio of the similarity of triangle a'b'c' to abc. The ratio area P' of a triangle a'b'c' to area P of the triangle abc is square of the ratio of the similarity. P' / P = k^2. Three methods for showing triangles to be similar. side - side - side, side - angle - side, angle - angle - angle.