5. (căn bậc hai 1/25  - x ) - căn bậc hai 1/81  =  -1/9

Lời giải:

\(5\,\,.\,\,\left( {\sqrt {\frac{1}{{25}}} - x} \right) - \sqrt {\frac{1}{{81}}} = - \frac{1}{9}\)

\(5\,\,.\,\,\left( {\frac{1}{5} - x} \right) - \frac{1}{9} = - \frac{1}{9}\)

\(5\,\,.\,\,\left( {\frac{1}{5} - x} \right) = - \frac{1}{9} + \frac{1}{9}\)

\(5\,\,.\,\,\left( {\frac{1}{5} - x} \right) = 0\)

\(\frac{1}{5} - x = 0\)

\(x = \frac{1}{5}\)

Vậy \(x = \frac{1}{5}\).