if $ b(x) <f(x) \leq g(x)$ where $b, g \in \Theta(x)$ can we state $f(x) \in \Theta(x)?$
b(x) is between $c_1·x$ and $c_2·x$ if x is large. g (x) is between $d_1·x$ and $d_2·x$ if x is large. So f (x) is between $c_1·x$ and $d_2·x$ if x is large, which makes it $\Theta(x)$.