1. Prove that the interior angles of a triangle have a sum of 180º
Objective
Prove that the interior angles of a triangle have a sum of 180º

2. Step 1
You need to remember that angles on a straight line add up to 180º.
Let’s call this Fact 1

3. a Step 2 You need to remember that alternate angles are equal.
Let’s call this Fact 2

4. Step 3
Draw two parallel lines

5. Step 4
Draw two lines within the parallel lines to create a triangle.

6. Step 5
Label the angles inside the triangle. a b c

7. Step 6
Now, use Fact 2 to label the alternate angles which are the same as b and c a b c b c

8. Step 7 Finally, notice that a, b and c form a straight line.
Fact 1 tells us that a + b + c = 180º

9. We have proved …
… that the interior angles in any triangle have a sum of 180º.