Triangles are polygons with three sides.
Types of Triangles
An acute − angled triangle contains no angles greater than 90°. e.g. A , B and C are all acute angles. 

A right − angled triangle contains an angle of 90°. e.g. A = 90° 

An obtuse − angled triangle contains an angle between 90°and 180°. 
 A scalene triangle contains three sides unequal in length. All angle sizes are different.
 An isosceles triangle is one in which two of the sides and their opposite angles are equal.
 An equilateral triangle is one in which all tree of the sides are equal and each angle is 60°.
 Congruent triangles have the same size and shape.
 Similar triangles have the same shape.
Symmetry of Triangles
An isosceles triangle has two equal sides and two equal angles .An isosceles triangle has one axis of symmetry. 
e.g. sides: a = b angles: A = B 

An equilateral triangle is one in which all three sides are of equal length and each interior angle is 60°. An equilateral triangle has three axes of symmetry. 
e.g. sides:
angles: 
Properties of Triangles
The interior or inside angles of a triangle always add up to 180°. They are supplementaryangles. 
a + b + c = 180°  
An exterior angle of a triangle is equal to the sum of the two opposite interior angles.  w = x + y  
The exterior angles of a triangle always add up to 360°.  m + n + p = 360° 
 The smallest angle of a triangle is opposite the smallest side.
 The largest angle of a triangle is opposite the largest side.