Cho hàm số \(f\left( x \right) = \frac{1}{{2x - 1}}.\) Tính \(f''\left( { - 1} \right).\) A. \( - \frac{8}{{27}}.\) B. \(\frac{2}{9}.\) C. \(\frac{8}{{27}}.\) D. \( - \frac{4}{{27}}.\)
Giải thích
Ta có \[f\left( x \right) = \frac{{0 \cdot x + 1}}{{2x - 1}} \Rightarrow f'\left( x \right) = \frac{{0 \cdot \left( { - 1} \right) - 2 \cdot 1}}{{{{\left( {2x - 1} \right)}^2}}} = - \frac{2}{{{{\left( {2x - 1} \right)}^2}}}\]
Suy ra \(f''\left( x \right) = {\left[ { - \frac{2}{{{{\left( {2x - 1} \right)}^2}}}} \right]^\prime } = \frac{{2 \cdot {{\left[ {{{\left( {2x - 1} \right)}^2}} \right]}^\prime }}}{{{{\left( {2x - 1} \right)}^4}}} = \frac{{8\left( {2x - 1} \right)}}{{{{\left( {2x - 1} \right)}^4}}} = \frac{8}{{{{\left( {2x - 1} \right)}^3}}}.\)
Khi đó \[f''\left( { - 1} \right) = \frac{8}{{{{\left[ {2 \cdot \left( { - 1} \right) - 1} \right]}^3}}} = \frac{8}{{{{\left( { - 3} \right)}^3}}} = - \frac{8}{{27}}.\]Chọn A.