\({4.9^x} - {13.6^x} + {9.4^x} = 0 \Leftrightarrow 4 - 13.{\left( {\frac{2}{3}} \right)^x} + 9{\left( {\frac{2}{3}} \right)^{2x}} = 0\)
Đặt \(t = {\left( {\frac{2}{3}} \right)^x},\) phương trình trở thành:
\(9{t^2} - 13t + 4 = 0 \Leftrightarrow \left[ \begin{array}{l}t = 1\\t = \frac{4}{9}\end{array} \right.\)
\( \Rightarrow \left[ {\begin{array}{*{20}{c}}{{{\left( {\frac{2}{3}} \right)}^x} = 1}\\{{{\left( {\frac{2}{3}} \right)}^x} = \frac{4}{9}}\end{array}} \right. \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{x = 0}\\{x = 2}\end{array}} \right. \Rightarrow T = 0 + 2 = 2\)
