Geometry Formulas and Rules for Triangles

By Mark Ryan

Part of Geometry Workbook For Dummies Cheat Sheet

The three-angled, two-dimensional pyramids known as triangles are one of the building blocks of geometry (however three-cornered they may be). Triangles, of course, have their own formulas for finding area and their own principles, presented here:


Triangles also are the subject of a theorem, aside from the Pythagorean one mentioned earlier. The Altitude-on-Hypotenuse Theorem makes dealing with triangles just a bit easier. It states that if you draw an altitude from the right angle of a right triangle to the hypotenuse, dividing the hypotenuse into two segments, then the altitude squared is equal to the product of the two segments of the hypotenuse. A leg of the right triangle squared is equal to the product of the segment of the hypotenuse nearer the leg and the entire hypotenuse.