# Derivative of Sine Function/Proof 4

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## Theorem

- $\map {D_x} {\sin x} = \cos x$

## Proof

\(\displaystyle D_x \left({\sin x}\right)\) | \(=\) | \(\displaystyle \lim_{h \mathop \to 0} \frac {\sin \left({x + h}\right) - \sin \left({x}\right)} h\) | Definition of Derivative of Real Function at Point | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \lim_{h \mathop \to 0} \frac {\sin \left({ \left({x + \frac h 2}\right) + \frac h 2}\right) - \sin \left({ \left({x + \tfrac h 2}\right) - \tfrac h 2}\right)} h\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \lim_{h \mathop \to 0} \frac {2 \cos \left({x + \frac h 2}\right) \sin \left({\frac h 2}\right)} h\) | Simpson's Formula for Cosine by Sine | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \lim_{h \mathop \to 0} \cos \left({x + \frac h 2}\right) \lim_{h \mathop \to 0} \frac{\sin \left({\frac h 2}\right)} {\frac h 2}\) | Multiple Rule for Limits of Functions and Product Rule for Limits of Functions | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \cos x \times 1\) | Continuity of Cosine and Limit of Sine of X over X | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \cos x\) |

$\blacksquare$