a: \(=x^2+3x+\dfrac{9}{4}-\dfrac{13}{4}=\left(x+\dfrac{3}{2}\right)^2-\dfrac{13}{4}>=-\dfrac{13}{4}\)
Dấu = xảy ra khi x=-3/2
b: \(=2\left(x^2-\dfrac{5}{2}x-2\right)\)
\(=2\left(x^2-2\cdot x\cdot\dfrac{5}{4}+\dfrac{25}{16}-\dfrac{57}{16}\right)\)
\(=2\left(x-\dfrac{5}{4}\right)^2-\dfrac{57}{8}>=-\dfrac{57}{8}\)
Dấu '=' xảy ra khi x=5/4