Which of the following is not a characteristic of all parallelograms?
a. Opposite sides are equal.
b. Adjacent angles are congruent.

Solution:

A parallelogram is a quadrilateral having two pairs of parallel and equal sides.

In the figure, one can observe that adjacent angles of a parallelogram are acute and obtuse respectively.

∠A or ∠BAD and ∠C or ∠BCD are obtuse, ∠B or ∠ABC, and ∠D or ∠ADC are acute.

Hence, adjacent angles in all the parallelograms are not congruent.

Which of the following is not a characteristic of all parallelograms?

Summary:

The characteristic which is not true for all parallelograms is 'adjacent angles are congruent'.