$\text{ B = }$ $x^{2}+x+1$
$\text{ B = }$ $x^{2}+2.$ $\frac{1}{2}$ $\text{ x + }$ $\frac{1}{4}+$ $\frac{3}{4}$
$\text{ B = ( x + }$ $\frac{1}{2})$$^{2}+$ $\frac{3}{4}$ $\geq$ $\frac{3}{4}$
$\text{ Vậy }$ $B_{min}=$ $\frac{3}{4}$
$\text{ <=> x = }$ $\frac{1}{2}$
$\text{ C = 2x }$$^{2}+6x$
$\text{ C = 2 (}$ $x^{2}+3x )$
$\text{ C = 2 ( }$ $x^{2}+2.x.$ $\frac{3}{2}+$ $\frac{9}{4}-$ $\frac{9}{4})$
$\text{ C = 2 (x + }$ $\frac{3}{2})$$^{2}-$ $\frac{9}{2}$ $\geq-$ $\frac{9}{2}$
$\text{ Vậy }$ $C_{min}= -$ $\frac{9}{2}$
$\text{ <=> x = -}$ $\frac{3}{2}$
