Explanation:
In an isosceles triangle, two sides are equal in
length. An isosceles triangle also has two angles of the same measure (Our solved example in mathguru.com uses this concept); namely, the angles opposite to the two sides of the
same length; 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.
A right triangle (or right-angled triangle, formerly called a rectangled triangle) has one of its interior
angles measuring 90° (a right angle). (Our solved example in mathguru.com uses this concept)
The side
opposite to the right angle is the hypotenuse; it is the longest
side of the right triangle.
http://en.wikipedia.org/wiki/Triangle
In geometry, two
figures are congruent if they have the same shape and size. More formally, two
sets of points are called congruent if, and only if, one can be transformed
into the other by an isometry,
i.e., a combination of translations, rotations and reflections.
The related concept of similarity permits a change in size.
Congruence
of triangles
Two triangles are congruent if their corresponding sides are equal in length and their
corresponding angles are equal in size.
If triangle ABC is congruent to triangle DEF, the relationship can
be written mathematically as:
In many cases it is sufficient to establish the equality of three
corresponding parts and use one of the following results to deduce the
congruence of the two triangles.
SAS (Side-Angle-Side): If two
pairs of sides of two triangles are equal in length, and the included angles are equal in measurement,
then the triangles are congruent. (Our solved
example in mathguru.com uses this concept)
http://en.wikipedia.org/wiki/Congruence_(geometry)
Supplementary angles
Supplementary
angles are pairs of angles that add up to 180 degrees. (Our solved example in mathguru.com uses this concept)
If the two
supplementary angles are adjacent (i.e. have a common vertex and share just one side), their non-shared
sides form a line. The supplement of an angle of 135 degrees is an angle of 45
degrees. The supplement of an angle of x degrees is an angle of
(180 − x) degrees. Supplementary angles do not have to
be on the same line, and can be separated in space. For example, adjacent
angles of a parallelogram are supplementary.
http://en.wikipedia.org/wiki/Supplementary_angles
The above explanation is copied from
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Commons Attribution- ShareAlike 3.0 Unported License.