Đáp án đúng: B
Giải chi tiết:\(\begin{array}{l}\,{\left( {3x - \frac{1}{2}} \right)^2} + \frac{{21}}{{25}} = 1\\\,\,\,\,\,{\left( {3x - \frac{1}{2}} \right)^2}\,\,\,\,\,\,\,\,\,\,\,\, = 1 - \frac{{21}}{{25}}\\\,\,\,\,\,\,{\left( {3x - \frac{1}{2}} \right)^2}\,\,\,\,\,\,\,\,\,\,\, = \frac{4}{{25}}\\\,\,\,\,\,\,\,\,{\left( {3x - \frac{1}{2}} \right)^2}\,\,\,\,\,\,\,\, = \,{\left( { \pm \frac{2}{5}} \right)^2}\end{array}\)
TH1:
\(\begin{array}{l}3x - \frac{1}{2} = \frac{2}{5}\\3x\,\,\,\,\,\,\,\,\,\, = \frac{2}{5} + \frac{1}{2} = \frac{9}{{10}}\\\,\,\,x\,\,\,\,\,\,\,\,\,\, = \frac{9}{{10}}:3\\\,\,\,x\,\,\,\,\,\,\,\,\,\, = \frac{3}{{10}}\end{array}\)
TH2:
\(\begin{array}{l}3x - \frac{1}{2} = - \frac{2}{5}\\3x\,\,\,\,\,\,\,\,\,\, = - \frac{2}{5} + \frac{1}{2} = \frac{1}{{10}}\\\,\,\,x\,\,\,\,\,\,\,\,\,\, = \frac{1}{{10}}:3\\\,\,\,x\,\,\,\,\,\,\,\,\,\, = \frac{1}{{30}}\end{array}\)
Vậy \(x = \frac{3}{{10}}\) hoặc \(x = \frac{1}{{30}}\).
Chọn B
