## Precalculus (6th Edition) Blitzer

$d=-6 \cos \dfrac{\pi}{2} t$
Amplitude, $|A|=-6$ Given: $Period =4$ and $f=\dfrac{\omega}{2 \pi}$ Now, period, $P=\dfrac{2 \pi}{\omega} \implies \omega =\dfrac{2 \pi}{4}=\dfrac{\pi}{2}$ So, $d=A \cos \omega t=-6 \cos \dfrac{\pi}{2} t$ Hence, $d=-6 \cos \dfrac{\pi}{2} t$