Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: https://www.khanacademy.org/math/geometry/hs-geo-analytic-geometry/hs-geo-parallel-perpendicular-eq/v/proof-that-perpendicular-lines-have-negative-reciprocal-slope
Sal proves that perpendicular lines have negative reciprocal slope using triangle similarity.
Watch the next lesson: https://www.khanacademy.org/math/high-school-math/math1/math1-analytic-geometry/math1-coordinate-plane-proofs/v/classfying-a-quadrilateral-on-the-coordinate-plane?utm_source=YT&utm_medium=Desc&utm_campaign=highschoolmath
Missed the previous lesson? https://www.khanacademy.org/math/high-school-math/math1/math1-analytic-geometry/math1-parallel-perpendicular-eq/v/proving-that-parallel-lines-have-the-same-slope?utm_source=YT&utm_medium=Desc&utm_campaign=highschoolmath
High School Math on Khan Academy: Did you realize that the word "algebra" comes from Arabic (just like "algorithm" and "al jazeera" and "Aladdin")? And what is so great about algebra anyway? This tutorial doesn't explore algebra so much as it introduces the history and ideas that underpin it.
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