Final Answer:
a) The figure is a parallelogram when X = Y.
b) The figure is a parallelogram when X = -Y.
Step-by-step explanation:
a) In a parallelogram, opposite angles are equal. So, for figure a, X= Y ensures that opposite angles are congruent, making the figure a parallelogram.
b) Similarly, in figure b, opposite angles are equal. Therefore, X = -Y ensures the equality of opposite angles, satisfying the condition for a parallelogram.
c) For figure c, adjacent angles in a parallelogram add up to 180 degrees. Hence, X + Y = 180 degrees ensures that the figure is a parallelogram by satisfying the angle-sum property.
d) In figure d, adjacent angles in a parallelogram add up to 360 degrees. Therefore, X + Y = 360 degrees ensures that the figure is a parallelogram by meeting the angle-sum criterion.