Question 220241
If their slopes multiply to -1, then they are perpendicular lines. Or another way to say it: Given a slope {{{m}}}, the perpendicular slope would be {{{-1/m}}}. So if one slope is the negative reciprocal of the other, then they are perpendicular.


Example: I claim that the lines {{{y=2x}}} and {{{y=-(1/2)x}}} are perpendicular. How do I prove this? Just multiply the two slopes {{{2}}} and {{{-1/2}}} to get {{{2(-1/2)=-2/2=-1}}}. So my claim is verified.