I need to find the formula of the function graphed below. The closest I've gotten is 5sin(-.78x)+5, but it doesn't quite match.
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The period is 8. So, $f(x) = 5-5\sin x$ has period $2\pi$. To change it to $8$, you need to multiply $x$ by $\dfrac{2\pi}{8} = \dfrac{\pi}{4}$. $f(x) = 5-5\sin\left( \dfrac{\pi}{4} x \right)$