`a)`

Điều kiện: `x != +-3`

`(9/(x^3-9x) + 1/(x+3)) : ((x-3)/(x^2+3x) - x/(3x+9))`

`= (9/(x(x-3)(x+3)) + (x(x-3))/(x(x-3)(x+3))) : ((3(x-3))/(3x(x+3)) - x^2/(3x(x+3)))`

`= (9+x^2-3x)/(x(x-3)(x+3)) : (3x-9-x^2)/(3x(x+3))`

`= (x^2-3x+9)/(x(x-3)(x+3)) : (-(x^2-3x+9))/(3x(x+3))`

`= (x^2-3x+9)/(x(x-3)(x+3)) * (3x(x+3))/(-(x^2-3x+9))`

`= 3/(-(x-3))`

`= -3/(x-3)`

`b)`

Điều kiện: `x !=-3; x != -2; x!= -1`

`(x+1)/(x+2)*((x+2)/(x+3):(x+3)/(x+1))`

`=(x+1)/(x+2)*((x+2)/(x+3)*(x+1)/(x+3))`

`=(x+1)/(x+2)*{(x+2)(x+1)}/(x+3)^2`

`=(x+1)^2/(x+3)^2`