[TEX]sqrt{5x^2+14x+9}-sqrt{x^2-x-20}=5sqrt{x+1}[/TEX]
Thế này đúng không nhỉ?
[tex]dk:x\geq 5[/tex]
[tex]\sqrt{5x^2+14x+9}-21-(\sqrt{x^2-x-20}-6)-5(\sqrt{x+1}-3)\Leftrightarrow (x-8)(\frac{x+10.8}{\sqrt{5x^2+14x+9}+21}-\frac{x+7}{\sqrt{x^2-x-20}+6}-\frac{5}{\sqrt{x+1}+3})[/tex]
[tex]\frac{x+7}{\sqrt{5x^2+14x+9}+21}< \frac{x+7}{\sqrt{x^2-x-20}+6}[/tex]
vì
[tex]\left\{\begin{\sqrt{5x^2+14x+9}> \sqrt{x^2-x-20}(dk:x\geq 5)} \\ {21>6}[/tex]
[tex]\frac{3,8}{\sqrt{5x^2+14x+9}+21}<\frac{5}{\sqrt{x+1}+3}[/tex]
vì
[tex]\left\{\begin{\sqrt{5x^2+14x+9}>\sqrt{x+1}(dk:x\ge 5)} \\ {21>3}[/tex]
Công từng vế
[tex]x=8[/tex]
(nghiệm duy nhất)