In a parallelogram, opposite angles are

In a parallelogram, opposite angles are

• A. Congruent
• B. Supplementary
• C. Complementary
• D. Unrelated to each other

Answer: (A)

Opposite angles in a parallelogram are congruent. This comes from the two pairs of opposite sides being parallel. Adjacent angles along a side are supplementary, so one pair of consecutive angles sums to 180 degrees and the next pair also sums to 180 degrees. Subtracting these equalities shows that the angle at one corner equals the angle at the opposite corner, and the same holds for the other pair. So opposite angles have equal measures. This isn’t describing them as generally supplementary or complementary; those properties don’t define opposite angles in a parallelogram, though a rectangle (a special parallelogram) has right angles, which are still congruent.

Opposite angles in a parallelogram are congruent. This comes from the two pairs of opposite sides being parallel. Adjacent angles along a side are supplementary, so one pair of consecutive angles sums to 180 degrees and the next pair also sums to 180 degrees. Subtracting these equalities shows that the angle at one corner equals the angle at the opposite corner, and the same holds for the other pair. So opposite angles have equal measures. This isn’t describing them as generally supplementary or complementary; those properties don’t define opposite angles in a parallelogram, though a rectangle (a special parallelogram) has right angles, which are still congruent.