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Triangles
Definitions and properties of triangles in geometry.
Angles of a Triangle
A triangle is a closed plane figure with three (straight) sides.
The sum of all three interior angles of a triangle is equal to 180 degrees.
Isosceles Triangle
A triangle with two sides equal in size.
Angles B and C are equal in size.
Equilateral Triangle
A triangle with all sides equal in size.
All angles of the triangle are equal in size.
Right Triangle
A triangle with a right angle.
c is the hypotenuse, a and b are the legs of the triangle.
Pythagorean theorem:
c
^{ 2}
= a
^{ 2}
+ b
^{ 2}
Sine law in a Triangle
Triangle without right angle may be solved using sine law expressed as follows
a / sin(A) = b / sin(B) = c / sin(C)
Cosine law in a Triangle
cosine law expressed as follows
a
^{ 2}
= b
^{ 2}
+ c
^{ 2}
- 2 b c cos(A)
or
b
^{ 2}
= a
^{ 2}
+ c
^{ 2}
- 2 a c cos(B)
or
c
^{ 2}
= b
^{ 2}
+ a
^{ 2}
- 2 b a cos(C)
More geometry references
Triangle Problems
Geometry Tutorials, Problems and Interactive Applets.