What can you say about the sum of an exterior angle of a triangle and its adjacent angle ? 
Look at the following diagram:
Interior and Exterior Angles are marked as , , &
You can also observe that:
∠ 1 = Exterior angle
∠ 2 = Adjacent Interior Angle to Exterior ∠ 1
Now, you can notice that ∠ 1 & ∠ 2
Are on same Vertex C
Share common Arm AC
Therefore, ∠ 1 & ∠ 2 are Adjacent Angles………………(Statement 1)
Also, ∠ 1 & ∠ 2 are on the same straight line BC……...(Statement 2)
So, from Statement 1 and Statement 2; we get
∠ 1 & ∠ 2 forms a linear pair.
Or we can also say that "Exterior angle of a triangle and its adjacent angle forms a Linear Pair".
Hence, sum of an exterior angle of a triangle and its adjacent angle is equals to 180 
Related Question Examples
What can you say about the sum of an exterior angle of a triangle and its adjacent angle ? 
Find measure of ∠ CBD in the following figure: 
Find the measure of adjacent interior angle of a triangle, whose exterior angles is 80° 
