a: \(=2\left(x^2-\dfrac{5}{2}x+\dfrac{3}{2}\right)\)
\(=2\left(x^2-2\cdot x\cdot\dfrac{5}{4}+\dfrac{25}{16}-\dfrac{1}{16}\right)\)
\(=2\left(x-\dfrac{5}{4}\right)^2-\dfrac{1}{8}\ge-\dfrac{1}{8}\)
Dấu '=' xảy ra khi x=5/4
b: \(=4\left(x^2+\dfrac{3}{4}x-\dfrac{1}{4}\right)\)
\(=4\left(x^2+2\cdot x\cdot\dfrac{3}{8}+\dfrac{9}{64}-\dfrac{25}{64}\right)\)
\(=4\left(x+\dfrac{3}{8}\right)^2-\dfrac{25}{16}\ge-\dfrac{25}{16}\)
Dấu '=' xảy ra khi x=-3/8
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