Which best explains why all equilateral triangles are similar?

Every equilateral triangle has three 60° angles, so any two have equal corresponding angles and are similar. A dilation can change one triangle’s side lengths to match the other’s, and a rigid transformation can then place it in the same position and orientation. Thus, combinations of dilations and rigid transformations map any equilateral triangle onto any other. Rigid transformations alone preserve side lengths, so they cannot map triangles of different sizes onto each other.

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