Select the single best definition that describes two perpendicular lines.
Two lines that intersect each other at an angle of 90°.
Two lines that have only one intersection.
Two lines whose product of gradients is equal to $1$1.
Two lines that have no intersections.
If two lines are perpendicular, what is the product of their gradients?
Are the lines $y=x+1$y=x+1 and $y=x-1$y=x−1 perpendicular?
Are the lines $y=3x-7$y=3x−7 and $y=-3x+6$y=−3x+6 perpendicular?