Đáp án:
3) \(\dfrac{{x + y}}{{x + y + z}}\)
Giải thích các bước giải:
\(\begin{array}{l}
3)\dfrac{{\left( {x - y} \right)\left( {x + y} \right)}}{{\left( {x - y} \right)\left( {x + y} \right) + z\left( {x - y} \right)}} = \dfrac{{\left( {x - y} \right)\left( {x + y} \right)}}{{\left( {x - y} \right)\left( {x + y + z} \right)}}\\
= \dfrac{{x + y}}{{x + y + z}}\\
4)\dfrac{{{{\left( {2x + 1} \right)}^2} - {{\left( {{x^2}} \right)}^2}}}{{\left( {x - 1} \right)\left( {x + 1} \right)}} = \dfrac{{\left( {2x + 1 - {x^2}} \right)\left( {2x + 1 + {x^2}} \right)}}{{\left( {x - 1} \right)\left( {x + 1} \right)}}\\
= \dfrac{{\left( { - {x^2} + 2x + 1} \right){{\left( {x + 1} \right)}^2}}}{{\left( {x - 1} \right)\left( {x + 1} \right)}}\\
= \dfrac{{\left( { - {x^2} + 2x + 1} \right)\left( {x + 1} \right)}}{{x - 1}}
\end{array}\)
