Three angles of a quadrilateral $ABCD$ are equal. Is it a parallelogram? Why or why not?
(B) No,it is not necessarily a parallelogram. $A$ quadrilateral is a parallelogram only if both pairs of opposite angles are equal. For example,if $\angle A = \angle B = \angle C = 80^{\circ}$,then the sum of angles in a quadrilateral is $360^{\circ}$,so $\angle D = 360^{\circ} - (80^{\circ} + 80^{\circ} + 80^{\circ}) = 120^{\circ}$. Since $\angle B \neq \angle D$ $(80^{\circ} \neq 120^{\circ})$,the condition for a parallelogram is not satisfied.
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