**Introduction**

We have learned that the angle sum of a triangle is . What about the quadrilaterals? The square and the rectangles have four right angles, so clearly, the sum of their interior angles is . But what about other quadrilaterals?

Before proceeding with the proof, to those who want to explore first, draw different types of quadrilaterals and use the protractor to measure their interior angles. What do you think is the angle sum of the quadrilaterals? Are the sums equal?

**Theorem**

The sum of the interior angles of a quadrilateral is .

**Proof**

Given quadrilateral , we have to show that the angle sum of the interior angles is .

Draw diagonal to divide the quadrilateral into two triangles. Let , , , , , and be the measures of the interior angles as shown on the figure below.

Using the triangle angle sum theorem, we know that and . But the sum of these six angle measures is the sum of the four interior angles of the quadrilateral (Can you see why?). Therefore, the sum of the interior angles of a quadrilateral is 360^{\circ}.

**Note to the Student**

In most countries, it is a convention (a standard) that angles should be named with Greek letters. I will not adopt that convention in this blog for convenience. I will be using letters in the English alphabet or even numbers to name angles. I believe that it is easier for you to learn if you are familiar with the symbols. On the other hand, if your teacher requires you to follow the convention, you should do so.

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