Objective

1
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.

Lets call this Fact 1
3
Step 2

• You need to remember that alternate angles are
  equal.

Lets 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

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.