Đáp án:
$\min M =-\dfrac{169}{4}\Leftrightarrow x =\dfrac{13}{2}$
Giải thích các bước giải:
$M = x^2 - 13x$
$\to M = x^2 - 2.\dfrac{13}{2}x +\dfrac{169}{4} -\dfrac{169}{4}$
$\to M = \left(x -\dfrac{13}{2}\right)^2 -\dfrac{169}{4}$
Ta có:
$\left(x -\dfrac{13}{2}\right)^2 \geq 0,\,\,\forall x$
$\to \left(x -\dfrac{13}{2}\right)^2 -\dfrac{169}{4} \geq -\dfrac{169}{4}$
$\to M \geq -\dfrac{169}{4}$
Dấu $=$ xảy ra $\Leftrightarrow x =\dfrac{13}{2}$
Vậy $\min M =-\dfrac{169}{4}\Leftrightarrow x =\dfrac{13}{2}$
