MCQ
If the function $f(x) = x^2 - kx + 5$ is increasing on $[2, 4]$, then:
  • A
    $\text{k}\in(2,\infty)$
  • B
    $\text{k}\in(-\infty,2)$
  • C
    $\text{k}\in(4,\infty)$
  • $\text{k}\in(-\infty,4)$

Answer

Correct option: D.
$\text{k}\in(-\infty,4)$
$f(x) = x^2 - kx + 5$
$f\ '(x) = 2x - k$
Given$:\ f(x)$ is increasing on $[2, 4].$
$\Rightarrow f\ '(x) > 0$
$\Rightarrow 2x - k > 0$
$\Rightarrow k < 2x$
$\because\ \text{x}\in[2,4],$ maximum value of $k$ is $4, k < 4.$
$\therefore\ \text{k}\in(-\infty,4)$

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