Đề bài

Thực hiện các phép cộng, trừ phân thức sau: a) \(\frac{{a - 3b}}{{a + b}} - \frac{{5a + b}}{{a + b}}\);          b) \(\frac{{7a - b}}{{2{a^3}}} + \frac{{b - 3a}}{{2{a^3}}}\); c) \(\frac{{{a^2}}}{{{{\left( {a - b} \right)}^2}}} - \frac{{{b^2}}}{{{{\left( {b - a} \right)}^2}}}\); d) \(\frac{{{a^2} + 3}}{{a - 2}} - \frac{{3a}}{{a - 2}} + \frac{{a - 1}}{{2 - a}}\).

Lời giải chi tiết

a) \(\frac{{a - 3b}}{{a + b}} - \frac{{5a + b}}{{a + b}} \) \(= \frac{{a - 3b - 5a - b}}{{a + b}} \) \(= \frac{{ - 4a - 4b}}{{a + b}} \) \(= \frac{{ - 4\left( {a + b} \right)}}{{a + b}} \) \(=  - 4\); b) \(\frac{{7a - b}}{{2{a^3}}} + \frac{{b - 3a}}{{2{a^3}}} \) \(= \frac{{7a - b + b - 3a}}{{2{a^3}}} \) \(= \frac{{4a}}{{2{a^3}}} \) \(= \frac{2}{{{a^2}}}\); c) \(\frac{{{a^2}}}{{{{\left( {a - b} \right)}^2}}} - \frac{{{b^2}}}{{{{\left( {b - a} \right)}^2}}} \) \(= \frac{{{a^2}}}{{{{\left( {a - b} \right)}^2}}} - \frac{{{b^2}}}{{{{\left( {a - b} \right)}^2}}} \) \(= \frac{{{a^2} - {b^2}}}{{{{\left( {a - b} \right)}^2}}} \) \(= \frac{{\left( {a - b} \right)\left( {a + b} \right)}}{{{{\left( {a - b} \right)}^2}}} \) \(= \frac{{a + b}}{{a - b}}\); d) \(\frac{{{a^2} + 3}}{{a - 2}} - \frac{{3a}}{{a - 2}} + \frac{{a - 1}}{{2 - a}} \) \(= \frac{{{a^2} + 3 - 3a - a + 1}}{{a - 2}} \) \(= \frac{{{a^2} - 4a + 4}}{{a - 2}} \) \(= \frac{{{{\left( {a - 2} \right)}^2}}}{{a - 2}} \) \(= a - 2\).