Proof of angle addition formula for sine | Trigonometry | Khan Academy

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https://www.khanacademy.org/math/trigonometry/less-basic-trigonometry/angle-addition-formula-proofs/e/understanding-angle-addition-formulas?utm_source=YT&utm_medium=Desc&utm_campaign=Trigonometry

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https://www.khanacademy.org/math/trigonometry/less-basic-trigonometry/angle-addition-formulas-trig/v/double-angle-formula-for-cosine-example-c?utm_source=YT&utm_medium=Desc&utm_campaign=Trigonometry

Trigonometry on Khan Academy: Big, fancy word, right? Don't be fooled. Looking at the prefix, tri-, you could probably assume that trigonometry ("trig" as it's sometimes called) has something to do with triangles. You would be right! Trig is the study of the properties of triangles. Why is it important? It's used in measuring precise distances, particularly in industries like satellite systems and sciences like astronomy. It's not only space, however. Trig is present in architecture and music, too. Now you may wonder...how is knowing the measurement and properties of triangles relevant to music?? THAT is a great question. Maybe you'll learn the answer from us in these tutorials!

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