The data points are $(1,7.9),(2,5.4),$ and $(3,-9)$ and the function form is $y=A \cos x+B \sin x$. So, we have the following equations:
\begin{align*}
7.9 &= A \cos 1 + B \sin 1 + \epsilon_1, \\
5.4 &= A \cos 2 + B \sin 2 + \epsilon_2, \\
-9 &= A \cos 3 + B \sin
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