Explanation:
Internal and external angle
Internal and External angles
In geometry, an interior angle (or internal
angle) is an angle formed by two sides of a polygon that share an endpoint. For a simple,
convex or concave polygon, this angle will be an angle on the 'inner side' of
the polygon. A polygon has exactly one internal angle per vertex.
If every internal angle of a simple, closed polygon is less than
180°, the polygon is called convex.
In contrast, an exterior
angle (or external angle) is an angle
formed by one side of a simple, closed polygon and a line extended from an
adjacent side.
The sum of the internal angle and the external angle on the same
vertex is 180°.
For example: x+35+75=180
x+110=180
x+110-110=180-110
x=70
http://en.wikipedia.org/wiki/Internal_and_external_angle
The measures of the interior angles of a triangle in Euclidean
space always add up to 180 degrees.^{ }This allows determination
of the measure of the third angle of any triangle given the measure of two
angles. An exterior angle of a triangle is an angle that is a linear pair (and hence supplementary)
to an interior angle. The
measure of an exterior angle of a triangle is equal to the sum of the measures
of the two interior angles that are not adjacent to it; this is the exterior
angle theorem. (Our solved example in mathguru.com uses this concept)
The sum of the measures of the three exterior angles (one for
each vertex) of any triangle is 360 degrees.
A
triangle, showing exterior angle d
^{ }
^{ }
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.