Câu `12`

`A=(1/(x+2)+5/(x-2)+4/(x^2-4)): 6/(x+3) (đk: xne 2;x ne -2;x ne -3)`

`A=[1/(x+2)+5/(x-2)+4/((x-2)(x+2))] . (x+3)/6`

`A=(x-2+5(x+2)+4)/((x-2)(x+2)) . (x+3)/6`

`A=(x-2+5x+10+4)/((x-2)(x+2)) . (x+3)/6`

`A=(6x+12)/((x-2)(x+2)) . (x+3)/6`

`A=(6(x+2))/((x-2)(x+2)) . (x+3)/6`

`A=(x+3)/(x-2)`

Câu `13`

`A=((7-2x)/(x-1)+(2x)/(x+1)-1/(x^2-1)): (3x+9)/(x^2-1) (đk: x ne 1;x ne -1;x ne -3)`

`A=[(7-2x)/(x-1)+(2x)/(x+1)-1/((x-1)(x+1))] . (x^2-1)/(3x+9)`

`A=((7-2x)(x+1)+2x(x-1)-1)/((x-1)(x+1)) . (x^2-1)/(3x+9)`

`A=(7x+7-2x^2-2x+2x^2-2x-1)/(x^2-1) . (x^2-1)/(3(x+3))`

`A=(3x+6)/(3(x+3))`

`A=(3(x+2))/(3(x+3))`

`A=(x+2)/(x+3)`