Answer
No, $\sin(2x)$ is not in the span of $f(x)$ and $g(x)$..
Work Step by Step
Since it is impossible to write $\sin(2x) = a\sin^2(x) + b\cos^2(x)$ for all $a,b \in \mathbb{R}$, $\sin(2x)$ is not in the span of $f(x)$ and $g(x)$.
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