If three parallel lines intersect two transversals, then they divide the transversals proportionally.

A Corollary

(a+b) / b = (c+d) / d

A Property

ad=bc

A Property

If a ray bisects an angle of a triangle, then it divides the opposite side into segments proportional to the other two sides.

A Theorem

If an angle of one triangle is congruent to an angle of another triangle and the sides including those angles are in proportion, then the triangles are similar