P=(x-1)(2x+3)
=2x2-2x+3x-3
=2x2+x-3
\(=2\left(x^2+\frac{1}{2}x+\frac{1}{16}-\frac{49}{16}\right)=2\left(x+\frac{1}{4}\right)^2-\frac{49}{8}\ge-\frac{49}{8}\)
dấu = xảy r khi x=-1/4
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