Prove and use slope criteria for parallel and perpendicular lines.
Determine whether two lines are perpendicular from their slopes
Problem
Are \(\text{two}\) lines with slopes \(3\) and \(-1/3\) perpendicular?
Big Picture
What this problem is really about
For two nonvertical lines, test perpendicularity by asking whether one slope is the negative reciprocal of the other. Multiplying the slopes is often the cleanest check: a product of negative one captures both the reciprocal size and the required sign change.
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