Tìm x biết 5 ⋅ ( √ 1/ 25 − x ) − √ 1 /81 = − 1 , 9 .

Lời giải:

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

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

\(5 \cdot \left( {\frac{1}{5} - {\rm{x}}} \right) = \left( { - 1,9} \right) + \frac{1}{9} = \frac{{ - 161}}{{90}}\)

\(\frac{1}{5} - {\rm{x}} = \frac{{ - 161}}{{90}}:5 = \frac{{ - 161}}{{450}}\)

\({\rm{x}} = \frac{1}{5} - \left( {\frac{{ - 161}}{{450}}} \right) = \frac{{251}}{{450}}\)

Vậy \({\rm{x}} = \frac{{251}}{{450}}\).