A right triangle (American English) or right-angled triangle (British English) is a triangle in which one angle is a right angle (that is, a 90degree angle). (Our solved example in mathguru.com uses this concept)

The relation between the sides and angles of a right triangle is the basis for trigonometry.

Right triangle

http://en.wikipedia.org/wiki/Right_triangle

In an isosceles triangle, two sides are equal in length. An isosceles triangle also has two angles of the same measure; namely, the angles opposite to the two sides of the same length; (Our solved example in mathguru.com uses this concept)

This fact is the content of the Isosceles triangle theorem. Some mathematicians define an isosceles triangle to have exactly two equal sides, whereas others define an isosceles triangle as one with at least two equal sides The latter definition would make all equilateral triangles isosceles triangles. The 45-45-90 Right Triangle, which appears in the Tetrakis square tiling, is isosceles.

http://en.wikipedia.org/wiki/Triangle

The above explanation is copied from Wikipedia, the free encyclopedia and is remixed as allowed under the Creative Commons Attribution- ShareAlike 3.0 Unported License.

(Our solved example in mathguru.com also uses the below concept.This is our own explanation, it is not taken from Wikipedia.)

The sum of all the angles in a Triangle is 180(Angle Sum Property)