Similar triangles:
This theory is based of the work of Thales, who used this concept to find the heights of the Anicent Pyramids. The idea is built off the fact that two triangles can be considered similar if their corresponding angles are congruent. If the sides are set to be congruent to one another then the corresponding side can be set up together in a proportion. Similar triangles are not the same shape but express similar size. A shape can be similar to one another through the idea that one shape may be an up scale or down scale of the other or a rotation.