The reason for the value of a+b being equal to d is because adjacent angles are supplementary.
First and foremost, it should be noted that a triangle has three sides and the sum of the angles in the triangle will be equal to 180°.
Therefore, based on the information given in the question, the solution will go thus:
a + b + c = 180°
Note that c + d = 180° since adjacent angles are supplementary.
Therefore, a + b + c = c + d
Collect like terms
a + b + c - c = d
a + b = d
Therefore, the equation has been proven and showed that a + b = d.
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