MCQ
From mean value theorem $f(b) - f(a) = $ $(b - a)f'({x_1});$   $a < {x_1} < b$ if $f(x) = {1 \over x}$, then ${x_1} = $
  • $\sqrt {ab} $
  • B
    ${{a + b} \over 2}$
  • C
    ${{2ab} \over {a + b}}$
  • D
    ${{b - a} \over {b + a}}$

Answer

Correct option: A.
$\sqrt {ab} $
a
(a) $f'({x_1}) = \frac{{ - 1}}{{x_1^2}}$

$\therefore \frac{{ - 1}}{{x_1^2}} = \frac{{\frac{1}{b} - \frac{1}{a}}}{{b - a}} = - \frac{1}{{ab}} $

$\Rightarrow {x_1} = \sqrt {ab} $.

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