Prove that sum of angle of triangle are 180
Consider a triangle ABC.
To prove, angle a + angle b + angle c = 180 degrees
Through vertex A, draw a line l parallel to BC.
Since alternate interior angles are equal so;
Angle b = angle x (1)
Angle c = angle y (2)
Since l is a straight line so, angle x + angle b + angle y = 180 degrees
Therefore, angle b + angle a + angle c = 180 degrees [Using (1) and (2)]
That is angle a + angle b + angle c = 180 degrees