\(=4\left(x^2+\dfrac{1}{4}x+\dfrac{5}{4}\right)\)
\(=4\left(x^2+2\cdot x\cdot\dfrac{1}{8}+\dfrac{1}{64}+\dfrac{79}{64}\right)\)
\(=4\left(x+\dfrac{1}{8}\right)^2+\dfrac{79}{16}>=\dfrac{79}{16}\)
Dấu '=' xảy ra khi x=-1/8
Có: \(4x^2+x+5\)
\(=4\left(x^2+\dfrac{1}{4}x+\dfrac{5}{4}\right)\)
\(=4\left(x^2+2.\dfrac{1}{8}x+\dfrac{1}{64}+\dfrac{79}{64}\right)\)
\(=4\left(x^2+2.\dfrac{1}{8}x+\dfrac{1}{64}\right)+\dfrac{79}{16}\)
\(=4\left(x+\dfrac{1}{8}\right)^2+\dfrac{79}{16}\ge\dfrac{79}{16}\)
Dấu ''='' xảy ra khi \(\left(x+\dfrac{1}{8}\right)^2=0\Rightarrow x=-\dfrac{1}{8}\)
