Types of triangles:
All sides have the same length. An equilateral triangle is also a regular polygon with all angles measuring 60°.
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.
All sides and internal angles are different from one another.
Triangles that do not have an angle that measures 90° are called oblique triangles.
A triangle that has all interior angles measuring less than 90° is an acute triangle or acute-angled triangle.
A triangle that has one angle that measures more than 90° is an obtuse triangle or obtuse-angled triangle.
A right triangle (or right-angled triangle) has one of its interior angles measuring 90° (a right angle).
The side opposite to the right angle is the hypotenuse; it is the longest side in the right triangle.
The other two sides are the legs or catheti of the triangle.
The sum of the squares of the lengths of the two legs is equal to the square of the length of the hypotenuse: a2 + b2 = c2, where a and b are the lengths of the legs and c is the length of the hypotenuse.