2.2 Definitions and Biconditional Statements. Definition. Two lines are called perpendicular lines if they intersect to form a right angle. A line perpendicular to a plane is a line that intersects the plane in a point and is perpendicular to every line in the plane that intersects it.
x = 3 if and only if x2 = 9.
No, because x also can be -3.
If two angles are supplementary, then the sum of their measures is 180°.
If the sum of two angles measure 180°, then they are supplementary.