117- The function $f(x) = \begin{cases} ax^3 + bx & ; \ x < 1 \\ 2\sqrt{4x - 3} & ; \ x \geq 1 \end{cases}$ is differentiable on the set of real numbers. What is $b$?
(1) $\dfrac{1}{2}$ \hfill (2) $1$ \hfill (3) $\dfrac{3}{2}$ \hfill (4) $2$
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