Menelaus Theorem (Transversal)

The transversal is parallel to a side of the triangle
Drag point D or point A. The two indicated ratios remain equal to each other. This way of showing equal ratios is often called Thales Theorem. It can be proved using the similar triangles ABC and ADE. The parallel transversal divides the two sides of triangle ABC proportionately. The next diagram in this sequence shows the case of a non-parallel transversal.