làm 1 câu cx đc.......................

`14062009`

`3.`

`a)` $\dfrac{2x - 4}{5}$ `+` $\dfrac{3x + 14}{5}$ `=` $\dfrac{(3x + 2x) + (14 - 4)}{5}$

`=` $\dfrac{5x  + 10}{5}$

`=` $\dfrac{5(x + 2)}{5}$

`= x + 2`

`-------------------`

`b)` $\dfrac{x + 1}{x - 5}$ `+` $\dfrac{x - 18}{x - 5}$ `+` $\dfrac{x + 2}{x - 5}$

`=` $\dfrac{(x + x  +x) + (1 - 18 + 2)}{x - 5}$

`=` $\dfrac{3x - 15}{x - 5}$

`=` $\dfrac{3(x - 5)}{x - 5}$

`= 3`

`-------------------`

`4.`

`a)` $\dfrac{6}{x^2 + 4x}$ `+` $\dfrac{3}{2x + 8}$

`=` $\dfrac{6}{x(x + 4)}$ `+` $\dfrac{3}{2(x + 4)}$

`=` $\dfrac{12}{2x(x + 4)}$ `+` $\dfrac{3x}{2x(x + 4)}$

`=` $\dfrac{3x + 12}{2x(x + 4)}$

`=` $\dfrac{3(x + 4)}{2x(x + 4)}$

`=` $\dfrac{3}{2x}$

`-------------------`

`b)` $\dfrac{x + 1}{x - 2}$ `+` $\dfrac{x - 2}{x + 2}$ `+` $\dfrac{x - 14}{x^2 - 4}$

`=` $\dfrac{x^2 + 3x + 2}{x^2 - 4}$ `+` $\dfrac{x^2 - 4x + 4}{x^2 - 4}$ `+` $\dfrac{x - 14}{x^2 - 4}$

`=` $\dfrac{(x^2 + x^2) + (3x + x - 4x) + (2 + 4 - 14)}{x^2 - 4}$

`=` $\dfrac{2x^2 - 8}{x^2 - 4}$

`=` $\dfrac{2(x^2 - 4)}{x^2 - 4}$

`= 2`

`-------------------`

`5.`

`a)` $\dfrac{4 - x^2}{x - 3}$ `+` $\dfrac{2x - 2x^2}{3 - x}$ `+` $\dfrac{5 - 4x}{x - 3}$

`=` $\dfrac{4 - x^2}{x - 3}$ `-` $\dfrac{2x - 2x^2}{x - 3}$ `+` $\dfrac{5 - 4x}{x - 3}$

`=` $\dfrac{x^2 - 6x + 9}{x - 3}$

`=` $\dfrac{(x - 3)^2}{x - 3}$

`= x - 3`

`-------------------`

`b)` $\dfrac{2}{x + 2}$ `+` $\dfrac{-4}{2 - x}$ `+` $\dfrac{5x + 2}{4 - x^2}$

`=` $\dfrac{2x - 4}{x^2 - 4}$ `-` $\dfrac{-4x + 8}{x^2 - 4}$ `-` $\dfrac{5x + 2}{x^2 - 4}$

`=` $\dfrac{x + 6}{x^2 - 4}$