a) \(\displaystyle {x \over { - 15}} = {{ - 60} \over x}\)
\(\Rightarrow x.x = \left( { - 15} \right).\left( { - 60} \right) \)
\(\Rightarrow {x^2} = 900\)
\( \Rightarrow {x^2} = {30^2}\)
\(\Rightarrow x = 30\) hoặc \(x = -30\).
b) \(\displaystyle {{ - 2} \over x} = {{ - x} \over {\displaystyle {8 \over {25}}}}\)
\(\displaystyle \Rightarrow - 2.{8 \over {25}} = x.\left( { - x} \right) \)
\(\displaystyle \Rightarrow - {{16} \over {25}} =- {x^2}\)
\( \displaystyle \Rightarrow {x^2} = {{16} \over {25}}\)
\( \Rightarrow {x^2} = {\left( {\dfrac{4}{5}} \right)^2}\)
\(\displaystyle \Rightarrow x = {4 \over 5}\) hoặc \(\displaystyle x = - {4 \over 5}\)
