Chọn câu sai:\(\frac{{(m - n)}}{{m + n}}:(m + n) = m - n\)

* Hướng dẫn giải

\(\begin{array}{l}
\frac{{{x^3} + {x^2} + x}}{x} = \frac{{x\left( {{x^2} + x + 1} \right)}}{x} = {x^2} + x + 1\\
\frac{{4{x^2} - 4{y^2}}}{{\left( {x + y} \right)\left( {x - y} \right)}} = \frac{{4\left( {{x^2} - {y^2}} \right)}}{{\left( {x + y} \right)\left( {x - y} \right)}} = \frac{{4\left( {x + y} \right)\left( {x - y} \right)}}{{\left( {x + y} \right)\left( {x - y} \right)}} = 4\\
\frac{{2x}}{5} + \frac{x}{3} - \frac{x}{2} = \frac{{12x}}{{30}} + \frac{{10x}}{{30}} - \frac{{15x}}{{30}} = \frac{{7x}}{{30}}\\
\frac{{\left( {m - n} \right)}}{{m + n}}:\left( {m + n} \right) = \frac{{\left( {m - n} \right)}}{{m + n}}.\frac{1}{{m + n}} = \frac{{m - n}}{{{{\left( {m + n} \right)}^2}}}
\end{array}\)

Chọn câu sai: D