The fifth proposition in Book I of Euclid’s *Elements* states that the two base angles of an isosceles triangle are equal (in the figure below, angles B and C).

For centuries, this result has been known as *Pons Asinorum*, or the Bridge of Asses, apparently a metaphor for a problem that separates bright sparks from dunces. Euclid proved the proposition by extending the sides AB and AC and drawing lines to form additional triangles. His proof is quite complicated. Continue reading ‘Pons Asinorum’