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Proof of Cosine Law

Natz

Sep 7, 2025

I want to walk you through a proof of the cosine law—but with a twist. We’re looking at a case where one of the angles is obtuse. This often feels tricky at first, but with the right tools and a bit of reflection, it becomes beautifully clear.I love this proof because it shows how math builds on itself. Even when things get less intuitive—like with obtuse angles—we can lean on what we already know. If you’ve ever felt unsure about the cosine law in non-right triangles, I hope this helped make it feel more approachable.

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Lesson

Mathematics

25 min
1,500 Points

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