Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
*)\\
\dfrac{x}{{ - 15}} = \dfrac{{ - 60}}{x}\\
\Leftrightarrow x.x = \left( { - 60} \right).\left( { - 15} \right)\\
\Leftrightarrow {x^2} = 60.15\\
\Leftrightarrow {x^2} = 4.15.15\\
\Leftrightarrow {x^2} = {2^2}{.15^2}\\
\Leftrightarrow {x^2} = {\left( {2.15} \right)^2}\\
\Leftrightarrow {x^2} = {30^2}\\
\Leftrightarrow \left[ \begin{array}{l}
x = 30\\
x = - 30
\end{array} \right.\\
*)\\
x:\left( { - 49} \right) = 4:\left( { - 28} \right)\\
\Leftrightarrow \dfrac{x}{{ - 49}} = \dfrac{4}{{ - 28}}\\
\Leftrightarrow \dfrac{x}{{ - 49}} = \dfrac{1}{{ - 7}}\\
\Leftrightarrow x = \left( { - 49} \right).\dfrac{1}{{ - 7}}\\
\Leftrightarrow x = 7\\
*)\\
0,36.4,25 = 0,91.7\\
\Leftrightarrow \left\{ \begin{array}{l}
\dfrac{{0,36}}{7} = \dfrac{{0,91}}{{4,25}}\\
\dfrac{{0,36}}{{0,91}} = \dfrac{7}{{4,25}}
\end{array} \right.
\end{array}\)
