Because one slope is positive while the other slope is a negative reciprocal of it.
For example: 2*-1/2 = -1
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If the product is -1 then the lines are perpendicular to one another.
Take any two lines and look at their slopes. -- If the slopes are equal, then the lines are parallel. -- If the product of the slopes is -1, then the lines are perpendicular.
You have to know the slopes of both lines. -- Take the two slopes. -- The lines are perpendicular if (one slope) = -1/(the other slope), or the product of the slopes equals to -1.
Are perpendicular to one another.
perpendicular