You can move the vertices (A, B, C) of the given triangle with pressed mouse button.

The sides of the triangle are extended to lines. In addition, the parallel to one side is drawn through the opposite vertex as an auxiliary line. The two angles marked in violet have the same size because they are alternate angles at parallel lines. Similarly, the angles drawn in green have the same size.

From the drawing, you can see that the three coloured angles next to each other (at vertex C) together form a straight angle (180°). Therefore, the sum of the measures of the three interior angles must also be 180°.