- Question:
- In the figure below, points A, B, C lie on a straight line, DBC is a triangle. If the angle ABD is 136
^{o}, what is the value of x?

- Solution:
- Since points A, B, and C lie on a straight line, so angle ABD is an exterior angle of triangle DBC
- angle ABD = angle D + angle C (Exterior angle of a triangle theorem)
- Given: angle D = x
^{o}and angle C = x^{o} - 136 = x + x
- 2x = 136
- x = 68
- The value of x is 68.

When extended one side of a triangle, an exterior angle of a triangle is formed. The exterior angle of a triangle is equal to the sum of two noadjacent interior angles.
In this problem, we get angle ABD = angle D + angle C. Given angle ABD is 136^{o}, then x + x = 136. 2x = 136. x = 68. So the value od x is 68.
That is, the angle C is equal to angle D which is 68^{o}.