Simple Similar Triangles

Drag points A, B, and C to change the size and shape of the blue triangle, and its white counterpart that is similar (constrained by proportional SAS). Drag the Red point D to change the ratio in sizes. Observe the multitude of calculated output lengths and angles, and how they match the proportion value, proving similarity, regardless of the triangles' shapes/sizes.

Tags: Triangles, Similar, Ratios, Draggable, Outputs,

Similar Triangle Incenter

This problem showed up in a math competition. See what you can make of it.  

Tags: incenter, circle, similar-triangles

Euclids Elements – Book 4 – Proposition 11

Can you create a regular pentagon without a protractor? Because it’s pretty impressive how Euclid did it with only a compass and a ruler. Note that the yellow triangle is similar to the one inside the pentagon.

Tags: Euclid, Elements, Geometry, Pentagon, Triangle