Sine of Sum/Proof 4
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Theorem
- $\map \sin {a + b} = \sin a \cos b + \cos a \sin b$
Proof
\(\ds \sin \left({a + b}\right)\) | \(=\) | \(\ds \cos \left({\frac \pi 2 - \left({a + b}\right)}\right)\) | Cosine of Complement equals Sine | |||||||||||
\(\ds \) | \(=\) | \(\ds \cos \left({\left({\frac \pi 2 - a}\right) - b}\right)\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \cos \left({\frac \pi 2 - a}\right) \cos b + \sin \left({\frac \pi 2 - a}\right) \sin b\) | Cosine of Difference | |||||||||||
\(\ds \) | \(=\) | \(\ds \sin a \cos b + \sin \left({\frac \pi 2 - a}\right) \sin b\) | Cosine of Complement equals Sine | |||||||||||
\(\ds \) | \(=\) | \(\ds \sin a \cos b + \cos a \sin b\) | Sine of Complement equals Cosine |
$\blacksquare$