Explanation:
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. The magnitude of
the angle is the "amount of rotation" that separates the two rays,
and can be measured by considering the length of circular arc swept out when
one ray is rotated about the vertex to coincide with the other (see
"Measuring angles", below). Where there is no possibility of
confusion, the term "angle" is used interchangeably for both the
geometric configuration itself and for its angular magnitude (which is simply a
numerical quantity).
∠, the angle symbol
http://en.wikipedia.org/wiki/Angle
Triangle
A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with
vertices A, B, and C is denoted ABC.
Triangle

A triangle

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 uses the below concept. This
is our own explanation, it is not taken from Wikipedia.)
The sum of all the angles of a Triangle
is 180(Angle Sum
Property of a Triangle)