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Now that we know the relationship between the slopes of parallel and perpendicular lines, we can use the slope formula to determine if lines are parallel or perpendicular. Watch this video, paying close attention to how he uses the slope formula to determine any relationships between pairs of lines.
Source: Khan Academy, https://www.khanacademy.org/math/geometry/hs-geo-analytic-geometry/hs-geo-parallel-perpendicular-lines/v/classify-lines This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.