MCQ
If the range of the function $f(x)=\frac{5-x}{x^{2}-3 x+2}$, $x \neq 1,2$, is $(-\infty, \alpha] \cup[\beta, \infty)$, then $\alpha^{2}+\beta^{2}$ is equal to :
  • A
    190
  • B
    192
  • C
    188
  • D
    194

Answer

D. 194
$y=\frac{5-x}{x^{2}-3 x+2}$
$\mathrm{yx}^{2}-3 \mathrm{xy}+2 \mathrm{y}+\mathrm{x}-5=0$
$y z^{2}+(-3 y+1) x+(2 y-5)=0$
Case I : If $\mathrm{y}=0$ (Accepted)
\begin{equation*}
\Rightarrow x=5
\end{equation*}
Case II : If $y \neq 0$
\begin{equation*}
\mathrm{D} \geq 0
\end{equation*}
$(-3 y+1)^{2}-4(y)(2 y-5) \geq 0$
$9 y^{2}+1-6 y-8 y^{2}+20 y \geq 0$
$y^{2}+14 y+1 \geq 0$
$(y+7)^{2}-48 \geq 0$
$|y+7| \geq 4 \sqrt{3}$
$\Rightarrow y+7 \geq 4 \sqrt{3}$ or $y+7 \leq-4 \sqrt{3}$
$\Rightarrow \mathrm{y} \geq 4 \sqrt{3}-7$ or $\mathrm{y} \leq-4 \sqrt{3}-7$
From Case I and Case II
$\mathrm{y} \in(-\infty,-4 \sqrt{3}-7] \cup[4 \sqrt{3}-7, \infty)$
So $\alpha=-4 \sqrt{3}-7$
\begin{equation*}
\begin{aligned}
\beta= & 4 \sqrt{3}-7 \\
\Rightarrow \mathrm{a}^{2}+\mathrm{b}^{2} & =(-4 \sqrt{3}-7)^{2}+(4 \sqrt{3}-7)^{2} \\
& =2(48+49) \\
= & 194
\end{aligned}
\end{equation*}

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