Question

prove that the sum of 3 sides of a triangle is 180°

Answer 1

its because of the angle sum property of a triangle.
this question must be "prove that the sum of 3 angles of a triangle is 180°"

Answer 2

Let ∆ABC be the triangle
Now draw a line XY through the point A parallel to the line BC
, ang. XAB+ ABC+ YAC =180°[straight line ] => 1
but Ang.XAB= ABC(alt. int. angle) [as XY||BC]=2
similarly ang. YAC= ACB. =>3
From 1,2 and 3
Ang. ABC+ BAC+ ACB=180°
Hence proved