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When the lines are horizontal and vertical. (slope of zero) (undefined slope)

Q: When is the product of the slope of two perpendicular lines not equal to -1?

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For any two perpendicular lines (save a vertical and a horizontal one), the product of their slopes is always -1. For two perpendicular lines with one having a slope of -2, the other will have a slope equal to -1 divided by -2, which equals 1/2.

When two lines are perpendicular to each other the product of their slopes is equal to -1.m1*m2 = -1

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.

Slopes of perpendicular lines will be opposite reciprocals. This means that the slopes have opposite signs and that one is 1/ the other. For example, 2 and -1/2.

Given any line L, with slope m, the perpendicular line has slope -m.

The slope of a perpendicular line is not defined.

if slope is given as m then perpendicular slope is -1/m (negative inverse)

Perpendicular, or "normal", means that two lines meet at right angles. The two lines cross in such a way that they form 4 equal angles between them.If you work with coordinates, the product of the slopes of two perpendicular lines are minus 1. For example, if a certain line has a slope of 2, another line, perpendicular to it, will have a slope of -1/2.

Two lines are parallel if and only if they have the same slope. Two lines are perpendicular if the product of their slopes is -1. If neither of these conditions are met, the lines are nether parallel, or perpendicular.

Perpendicular lines have slopes whose product is -1. As this is always true, if we think of .33 as about 1/3, then the perpendicular line would have a slope of -1/(1/3) which is -3.

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If the angle formed between the intersecting lines are 90o then the two lines are perpendicular. In 2D coordinate geometry, a perpendicular line has a slope equal to the negative reciprocal of the original line.