Geometry
M. Harvey - the book
UPDATE: I have more or less finished the first three parts now and the last part is well underway.
Geom Lessons/Demos
Geometry Application
GeoGebra
Indirect Proof. How to write indirect proofs that employ proof by contradiction: for triangles, segments, perpendicular lines
Indirect proof is synonymous with proof by contradiction. A keyword signalling that you should consider indirect proof is the word 'not'.On a sphere, the sum of the angles of a triangle is not equal to 180°. A sphere is not a Euclidean space, but locally the laws of the Euclidean geometry are good approximations. In a small triangle on the face of the earth, the sum of the angles is very nearly 180.
Spherical geometry
Geometry
T he word geometry is Greek for geos - meaning earth and metron - meaning measure. Geometry was extremely important to ancient societies and was used for surveying, astronomy, navigation, and building. Geometry, as we know it is actually known as Euclidean geometry which was written well over 2000 years ago in Ancient Greece by Euclid, Pythagoras, Thales, Plato and Aristotle just to mention a few.