Theorem 15

A line drawn parallel to one side of a triangle divides the other two sides in the same ratio.
ABC is a triangle. The point, P, is on the side [AB]. You can move this point. The line through P is parallel to [BC] and meets [AC] at Q. As you change the triangle and the point P you can see that the line PQ divides the two sides, [AB] and [AC], in the same ratio.