Lösung 1.1:2f
Aus Online Mathematik Brückenkurs 2
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Version vom 10:00, 11. Mär. 2009
We can rewrite the function using a trigonometric addition formula,
| \displaystyle f(x) = \cos\Bigl(x+\frac{\pi}{3}\Bigr) = \cos x\cdot\cos \frac{\pi}{3} - \sin x\cdot\sin\frac{\pi}{3}\,\textrm{.} |
If we now differentiate this expression, \displaystyle \cos (\pi/3) and \displaystyle \sin (\pi/3) are constants and we obtain
| \displaystyle \begin{align}
f^{\,\prime}(x) &= \frac{d}{dx}\,\Bigl(\cos x\cdot\cos\frac{\pi}{3} - \sin x\cdot\sin\frac{\pi}{3} \Bigr)\\[5pt] &= \cos\frac{\pi}{3}\cdot\frac{d}{dx}\,\cos x - \sin\frac{\pi}{3}\cdot\frac{d}{dx}\,\sin x\\[5pt] &= \cos\frac{\pi}{3}\cdot (-\sin x) - \sin\frac{\pi}{3}\cdot\cos x\,\textrm{.} \end{align} |
If we then use the addition formula in reverse, this gives
| \displaystyle \begin{align}
f^{\,\prime}(x) &= -\Bigl(\sin x\cdot\cos\frac{\pi}{3} + \cos x\cdot\sin\frac{\pi}{3}\Bigr)\\[5pt] &= -\sin\Bigl(x+\frac{\pi}{3}\Bigr)\,\textrm{.} \end{align} |
Note: In the next section, we will go through differentiation rules which make it possible to differentiate the expression directly without rewriting in this way.
