Two geometrical figures are similar if they are of the same shape but not necessarily of the same size. This includes the case when one is a mirror-image of the other, so the three triangles shown in the figure are all similar. For two similar triangles, there is a correspondence between their vertices such that corresponding angles are equal and the ratios of corresponding sides are equal. In the figure, ∠A = ∠P, ∠B = ∠Q, ∠C = ∠R and QR/BC = RP/CA = PQ/AB = 3/2.
Three similar triangles.