Đề bài

Tính nhanh \(\left( {\frac{y}{{x + z}} - \frac{y}{{x + y}} + \frac{{x + y}}{{x + y + z}}} \right) - \left( {\frac{y}{{x + z}} - \frac{z}{{x + y + z}} + \frac{x}{{x + y}}} \right).\)

Lời giải chi tiết

Ta có \(\begin{array}{l}\left( {\frac{y}{{x + z}} - \frac{y}{{x + y}} + \frac{{x + y}}{{x + y + z}}} \right) - \left( {\frac{y}{{x + z}} - \frac{z}{{x + y + z}} + \frac{x}{{x + y}}} \right)\\ = \frac{y}{{x + z}} - \frac{y}{{x + y}} + \frac{{x + y}}{{x + y + z}} - \frac{y}{{x + z}} + \frac{z}{{x + y + z}} - \frac{x}{{x + y}}\\ = \left( {\frac{y}{{x + z}} - \frac{y}{{x + z}}} \right) + \left( { - \frac{y}{{x + y}} - \frac{x}{{x + y}}} \right) + \left( {\frac{{x + y}}{{x + y + z}}\frac{z}{{x + y + z}}} \right)\\ = 0 + \frac{{ - \left( {x + y} \right)}}{{x + y}} + \frac{{x + y + z}}{{x + y + z}} = \left( { - 1} \right) + 1 = 0\end{array}\)