Prove that the sum of the angles of a quadrilateral is 360°.
ABCD is a quadrilateral and AC is a diagonal. Now, we get two triangles
∆ ABC and ∆ ACD.
As we know that sum of angles of triangle is 180°
So, sum of two triangles will be 180° × 2 = 360°
i.e. the sum of the angles of a quadrilateral is 360°