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07 Parallel and perpendicular lines When two lines (in the Cartesian plane) do not intersect (that is, they have no points in common), they are said to be parallel. Two non-vertical lines are parallel if and only if their slopes are equal and they have different y-intercepts. And Vertical lines are parallel if they have different x-intercepts Two lines are parallel if and only if they have the same slope and different y-intercepts. Ex. 1.) determine whether two lines are parallel. Two lines that intersect at a right angle (90 ), are perpendicular. Two non-vertical lines are perpendicular if and only if the product of their slopes is -1. Alternatively, two non-vertical are perpendicular if their slopes are negative reciprocals of each other. Any vertical line is perpendicular to any horizontal line. Find the slope of any line that is perpendicular to a line whose slope is 5/4 = -4/5 because: Ex.4) a.)