B=2(x^2+2.x.1/4 +1/16)^2 -57/8
=2.(x+1/4)^2 -57/8
MinB=-57/8 khi x=-1/4
\(B=-14+2x^2+x=2\left(x^2+\dfrac{1}{2}x+\dfrac{1}{16}\right)-\dfrac{113}{8}=2\left(x+\dfrac{1}{4}\right)^2-\dfrac{113}{8}\ge-\dfrac{113}{8}\)\(ĐTXR\Leftrightarrow x=-\dfrac{1}{4}\)
\(B=2x^2+x-14\)
\(=2\left(x^2+\dfrac{1}{2}x-7\right)\)
\(=2\left(x^2+2\cdot x\cdot\dfrac{1}{4}+\dfrac{1}{16}-\dfrac{113}{16}\right)\)
\(=2\left(x+\dfrac{1}{4}\right)^2-\dfrac{113}{8}\ge-\dfrac{113}{8}\forall x\)
Dấu '=' xảy ra khi \(x=-\dfrac{1}{4}\)
