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if they are parallel they run side by side forever and will never cross. if they are perpendicular they will cross at a 90 degree angle.

You can also tell just by looking at the equations for the lines if they are in the slope-intercept form (y = mx + b, where m is the slope and b is the y-intercept). When two lines are parallel, they have the same slope. When two lines are perpendicular, the slope of one is the negative reciprocal of the slope of other. For example, a line with a slope of 2 is perpendicular to a line with a slope of -½, and a line with a slope of 1 is perpendicular to a line with a slope of -1. (y = 1 and x = 1 are perpendicular because the slope of y = 1 is zero, the slope of x = 1 is infinity, the reciprocal of infinity is zero, and negative zero equals zero.)

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Q: How do you know if two graphed lines are parallel or perpendicular?
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What is parallel perpendicular lines look like?

As far as we know, two lines can be perpendicular, or they can be parallel, but they can't be both.

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