In a parallelogram, two adjacent angles are supplementary.

Explanation

To prove: In a parallelogram, two adjacent angles are supplementary.
Let ABCD be a parallelogram.
Then `AD ∣∣ BC` & `AB` is transversal.
`therefore A+B=180^@` [since, sum of the interior angles on the same since of the transversal is `180^@`]
Similarly,
`∠B+∠C=180^@`, `∠C+∠D=180^@` and `∠D+∠A=180^@`
Thus, the sum of any `2` adjacent angles of a parallelogram is `180^@`.
Hence, any `2` adjacent angles of a parallelogram are supplementary.