Question 832275
One way to show this, not necessarily as a formal proof, is create or construct a triangle on cardboard, cut out the triangle figure neatly, tear or cut into three pieces so that each piece contains one vertex.  Arrange the vertices on a flat surface so that they meet in one point and neighboring proximate sides of the pieces are all in contact.  You will find the side of the figure with the connected vertices at one point will form a straight angle.  The straight angle means the angles so arranged are supplementary in the arrangement, so 180 degrees.