If the sides of two triangles are in proportion, then the trianlges are similar.

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

A Theorem

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

A Property

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