Simply and shortly how do I show that this $33 = 33 + 5 \cos(720\cdot t)$ is equal to this $720 \cdot t = 90.$
Thank you for your help.
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$\begingroup$ Well if $0=5\cos (720t)$ then $\cos (720t) = 0$ and since $\cos (90) = 0$ it can be said that $720t = 90$. Note: This is actually not quite correct as there are infinite $\theta$ such that $\cos (\theta) = 0$, ie. -90,90,270 etc... $\endgroup$– KSabCommented Apr 14, 2014 at 9:59
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$$ 33 = 33 + 5\cos(720t) $$
$$ \Leftrightarrow \cos(720t) = 0 $$
$$ \Leftrightarrow 720t = \frac{\pi}{2} + k\pi = 90 ^{\circ} + k \cdot 180^{\circ} \qquad k \in \mathbb{Z}$$
Simply choose $k = 0$
