Final answer:
When a quadrilateral is divided into two triangles by drawing a diagonal, the sum of the measures of the angles in the quadrilateral is 360°.
Step-by-step explanation:
When a quadrilateral is divided into two triangles by drawing a diagonal, we can use the angle sum property of a triangle to argue that the sum of the measures of angle A, angle B, angle C, and angle D amounts to 180° + 180° = 360°. This is because each triangle has angles that add up to 180°, and since there are two triangles, the total sum is 180° + 180° = 360°.