Cho hàm số f ( x ) = { a x 2 + b x k h i x ≥ 1 2 x − 1 k h i x < 1 . Tìm a, b để hàm số có đạo hàm tại x = 1.

\[\mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} {\rm{f}}\left( {\rm{x}} \right) = \mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} \left( {{\rm{a}}{{\rm{x}}^{\rm{2}}}{\rm{ + bx}}} \right){\rm{ = a + b = f}}\left( {\rm{1}} \right)\]

\[\mathop {\lim }\limits_{{\rm{x}} \to {1^ - }} {\rm{f}}\left( {\rm{x}} \right) = \mathop {\lim }\limits_{{\rm{x}} \to {1^ - }} \left( {{\rm{2x}} - 1} \right) = 1\]

Để hàm số liên tục tại x = 1 thì \[\mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} {\rm{f}}\left( {\rm{x}} \right) = \mathop {\lim }\limits_{{\rm{x}} \to {1^ - }} {\rm{f}}\left( {\rm{x}} \right){\rm{ = f}}\left( {\rm{1}} \right) \Leftrightarrow {\rm{a + b = 1}}\,\,\,\left( {\rm{1}} \right)\]

Khi đó ta có: \[{\rm{f'}}\left( 1 \right) = \mathop {\lim }\limits_{x \to 1} \frac{{{\rm{f}}\left( {\rm{x}} \right) - {\rm{f}}\left( {\rm{1}} \right)}}{{{\rm{x}} - {\rm{1}}}}\]

\[\mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} \frac{{{\rm{f}}\left( {\rm{x}} \right) - {\rm{f}}\left( {\rm{1}} \right)}}{{{\rm{x}} - {\rm{1}}}} = \mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} \frac{{{\rm{a}}{{\rm{x}}^{\rm{2}}}{\rm{ + bx}} - \left( {{\rm{a + b}}} \right)}}{{{\rm{x}} - {\rm{1}}}} = \mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} \frac{{{\rm{a}}\left( {{{\rm{x}}^{\rm{2}}} - {\rm{1}}} \right){\rm{ + b}}\left( {{\rm{x}} - {\rm{1}}} \right)}}{{{\rm{x}} - {\rm{1}}}}\]

\[ = \mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} \left[ {{\rm{a}}\left( {{\rm{x + 1}}} \right){\rm{ + b}}} \right] = {\rm{2a + b}}\]

\[\mathop {\lim }\limits_{{\rm{x}} \to {1^ - }} \frac{{{\rm{f}}\left( {\rm{x}} \right) - {\rm{f}}\left( {\rm{1}} \right)}}{{{\rm{x}} - {\rm{1}}}} = \mathop {\lim }\limits_{x \to {1^ - }} \frac{{{\rm{2x}} - {\rm{1}} - \left( {{\rm{a + b}}} \right)}}{{{\rm{x}} - 1}} = \mathop {\lim }\limits_{{\rm{x}} \to {1^ - }} \frac{{2{\rm{x}} - 2}}{{{\rm{x}} - 1}} = 2\]

Để hàm số có đạo hàm tại x = 1 thì

\[{\rm{f'}}\left( 1 \right) = \mathop {\lim }\limits_{{\rm{x}} \to {1^ + }} \frac{{{\rm{f}}\left( {\rm{x}} \right) - {\rm{f}}\left( 1 \right)}}{{{\rm{x}} - 1}} = \mathop {\lim }\limits_{{\rm{x}} \to {1^ - }} \frac{{{\rm{f}}\left( {\rm{x}} \right) - {\rm{f}}\left( 1 \right)}}{{{\rm{x}} - 1}} \Leftrightarrow {\rm{2a + b}} = 2\,\,\,\left( 2 \right)\]

Từ (1) và (2) ta có hệ: \(\left\{ {\begin{array}{*{20}{c}}{a + b = 1}\\{2a + b = 2}\end{array} \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{a = 1}\\{b = 0}\end{array}} \right.} \right.\)

Đáp án cần chọn là: C