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C ______________\______________ D
In the above illustration angle AOC and angle AOD form a linear pair.
The pair is said to be linear because the sum of the angles is 180 degrees.
You can tell that their sum is 180degrees as the points C , O and D are collinear ie they lie in the same straight line. So angle COD = 180 degrees.
Therefore angle AOC + angle AOD = 180 degrees. You can say that angle AOC and angle AOD are adjacent angles . all linear pairs can be called adjacent angles
but all adjacent angles need not be linear pairs. It is not necessary that the sum of two adjacent angles to be equal to 180 degrees.