In geometry, the definition of a triangle is a two-dimensional closed figure with three sides. Aside from this, it is proven that the sum of the angles of a triangle is 180°. This is a triangle's innate property. Therefore, any closed shape with three sides is a triangle and it follows that the sum of the three angles is equal to 180°.
It is logical therefore, that the first step in proving this is by using the definition of a triangle.