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} = $

  • A

    $\sqrt {ab} $

  • B

    ${{a + b} \over 2}$

  • C

    ${{2ab} \over {a + b}}$

  • D

    ${{b - a} \over {b + a}}$

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