MCQ
If $f(x)$ is continuous on $[-4,2]$, where $f(x)=\left\{\begin{array}{ll}6 b-3 a x, & \text { for }-4 \leq x<-2 \\ 4 x+1, & \text { for }-2 \leq x \leq 2\end{array}\right.$, then $a+b=$
  • A
    $\frac{1}{6}$
  • B
    $-\frac{1}{6}$
  • C
    $\frac{7}{6}$
  • $-\frac{7}{6}$

Answer

Correct option: D.
$-\frac{7}{6}$
(D)
Since $f(x)$ is continuous on $[-4,2]$.
$\therefore \quad$ it is continuous at $x=-2$.
$\therefore \quad \lim _{x \rightarrow-2^{-}} f (x)=\lim _{x \rightarrow-2^{+}} f (x)$
$\Rightarrow \lim _{x \rightarrow-2^{-}}(6 b-3 a x)=\lim _{x \rightarrow-2^{+}}(4 x+1)$
$\Rightarrow 6 b-3 a (-2)=4(-2)+1$
$\Rightarrow 6 b+6 a=-7$
$\Rightarrow a + b =-\frac{7}{6}$

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