Giải thích các bước giải:
$\begin{split}6x^2-5x-40&=6(x^2-\dfrac{5x}{6})-40\\&=6(x^2-2x.\dfrac{5}{12}+(\dfrac{5}{12})^2-(\dfrac{5}{12})^2)-40\\&=6((x-\dfrac{5}{12})^2-(\dfrac{5}{12})^2)-40\\&=6(x-\dfrac{5}{12})^2-\dfrac{985}{24}\\&=6((x-\dfrac{5}{12})^2-\dfrac{985}{144})\\&=6(x-\dfrac{5}{12}-\sqrt{\dfrac{985}{144}})(x-\dfrac{5}{12}+\sqrt{\dfrac{985}{144}})\\&=6(x-\dfrac{5+\sqrt{985}}{12})(x-\dfrac{5-\sqrt{985}}{12})\end{split}$
