hoặc sd cách này dễ hơn:
[tex]B=\frac{2010,2011x^2-2x+2012,2013}{2014,2015x^2}[/tex]
[tex]B=\frac{2012,2013}{2014,2015}.\frac{1}{x^2}-\frac{2}{2014,2015}.\frac{1}{x}+\frac{2010,2011}{2014,2015}[/tex]
Đặt [tex]a=\frac{2012,2013}{2014,2015}, [/tex]
[tex]b=-\frac{2}{2014,2015},[/tex]
[tex]c=\frac{2010,2011}{2014,2015}[/tex]
Ta có: [tex]B_{min}=\frac{4ac-b^2}{4a}[/tex] tại [tex]\frac{1}{x}=\frac{-b}{2a} \Rightarrow x=?[/tex]