$\angle A = \angle B$. If $\angle A$ and $\angle B$ are supplementary angles,then $\angle A = \ldots$ (in $^o$)

Prove that two lines that are respectively perpendicular to two intersecting lines intersect each other.

In $\Delta ABC$,$\angle A + \angle B = 80^{\circ}$ and $\angle B + \angle C = 150^{\circ}$. Find the measure of each angle of $\Delta ABC$.

Can a triangle have all angles less than $60^{\circ}$? Give reason for your answer.

If the measure of an acute angle is $x^{\circ}$,find the difference of the measure of its supplementary angle and complementary angle. (in $^{\circ}$)

If one of the two angles of a linear pair has measure $75^{\circ}$,the measure of the other is $\ldots \ldots \ldots$ (in $^{\circ}$)