[tex]\frac{1}{4}(5-\frac{1}{5^{99}})x+\frac{1}{4\times 5^{99}}x=\frac{1}{25}(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{380})[/tex]
[tex]\Leftrightarrow \frac{5}{4}x-\frac{x}{4\times5^{99}}+\frac{x}{4\times 5^{99}}=\frac{1}{25}(\frac{1}{1\times 2}+\frac{1}{2\times 3}+...+\frac{1}{19\times 20})[/tex]
[tex]\Leftrightarrow \frac{5}{4}x=\frac{1}{25}(1-\frac{1}{20})[/tex]
[tex]\Leftrightarrow \frac{5}{4}x=\frac{1}{25}\times \frac{19}{20}[/tex] [tex]\Leftrightarrow x=\frac{1}{25}\times\frac{19}{20}\times \frac{4}{5}=\frac{19}{625}[/tex]
Vậy nghiệm của phương trình là [TEX]x=\frac{19}{625}[/TEX]