Question

[Ngô Hồng Thuận]

Tìm x sao cho biểu thức: \(\left(2x-3\right)\left(4+3x\right)\) đạt giá trị nhỏ nhất.

Answer 1

[Ngọc Vĩ]

\(=8x+6x^2-12-9x\)

\(=6x^2-x-12=\left(-6\right)\left(-x^2+\frac{1}{6}x+2\right)\)

\(=\left(-6\right)\left[-x^2-2.\frac{1}{12}.\left(-x\right)+\left(\frac{1}{12}\right)^2-\left(\frac{1}{12}\right)^2+2\right]\)

\(=\left(-6\right)\left[\left(-x-\frac{1}{12}\right)^2+\frac{287}{144}\right]\)

\(=\left(-6\right)\left(-x-\frac{1}{12}\right)^2-\frac{287}{24}\ge-\frac{287}{24}\)

Vậy Min biểu thức = \(-\frac{287}{24}\) khi \(\left(-x-\frac{1}{12}\right)^2=0\Rightarrow-x-\frac{1}{12}=0\Rightarrow-x=\frac{1}{12}\Rightarrow x=-\frac{1}{12}\)