HG 39

D23. Two polygons p, q are called equivalent if and only if they can be subdivided into the same finite number of triangles p_I , q_I, I = 1, 2, 3, . . . . which are congruent in pairs, p₁ to q₁ , p₂ to q₂ , and so on.

T65. A triangle is equivalent to each of its associated Saccheri quadrilaterals.

T66. If two triangles or quadrilaterals are congruent, then they are equivalent.

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