Triangles are polygons with three sides.
Angle Properties of Triangles
The interior angles of a triangle always add up to 180°. ( ∠ sum of Δ) 
a + b + c = 180°


An exterior angle of a triangle is equal to the sum of the two opposite interior angles. (ext. ∠ of Δ = sum of int.opp ∠s) 
w = x + y


The exterior angles of a triangle always add up to 360°. (sum of ext. ∠s of Δ)

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.
Types of Triangles
An acute − angled triangle contains no angles greater than 90°

∠A , ∠B and ∠C are all acute angles.


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

∠A = 90°


An obtuse − angled triangle contains an angle between 90°and 180°.  ∠A is an obtuse angle  
An isosceles triangle has one axis of symmetry. An isosceles triangle has two equal sides and two equal angles. 
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.

sides: a = b = c angles: 