\(\dfrac{4x-5}{5x+7}=\dfrac{4x-7}{5x+9}\)
\(\Rightarrow\left(4x-5\right)\left(5x+9\right)=\left(4x-7\right)\left(5x+7\right)\)
\(\Rightarrow20x^2-11x-45=20x^2+7x-49\)
\(\Rightarrow-11x-45=7x-49\)
\(\Rightarrow4=18x\Rightarrow x=\dfrac{9}{2}\)
Vậy...
ta có : \(\dfrac{4x-5}{5x+7}=\dfrac{4x-7}{5x+9}\Leftrightarrow\left(4x-5\right)\left(5x+9\right)=\left(5x+7\right)\left(4x-7\right)\)
\(\Leftrightarrow20x^2+36x-25x-45=20x^2-35x+28x-49\)
\(\Leftrightarrow20x^2+36x-25x-45-20x^2+35x-28x+49=0\)
\(\Leftrightarrow18x+4=0\Leftrightarrow18x=-4\Leftrightarrow x=\dfrac{-4}{18}=\dfrac{-2}{9}\) vậy \(x=\dfrac{-2}{9}\)
\(\dfrac{4x-5}{5x+7}=\dfrac{4x-7}{5x+9}\)
\(\Rightarrow\left(4x-5\right)\left(5x+9\right)=\left(4x-7\right)\left(5x+7\right)\)
\(\Leftrightarrow20x^2+36x-25x-45=20x^2+28-35x-39\)
\(\Leftrightarrow20x^2+11x-45=20x^2-7x-49\)
\(\Leftrightarrow20x^2+11x-45-20x^2+7x+49=0\)
\(\Leftrightarrow18x+4=0\)
\(\Rightarrow x=-\dfrac{4}{18}\)
