Let's list the pairs of alternate interior angles from the figure!
∠DAC and∠ACB ∠ABD and ∠BDC.
b
Angles that are supplementary have measures that add up to 180^(∘). If we ignore the diagonals, we can locate two pairs of consecutive interior angles if we view AD and BC as transversals to AB and DC. Because these sides are parallel, we know by the Consecutive Interior Angles Theorem, that they are supplementary.
The supplementary pairs of angles from the figure are.
∠DAB and ∠ADC ∠ABC and ∠DCB
