4, Rút gọn biểu thức

`+)` Với `x>0; x ne 1; x ne 4,` ta có:

     `B= ((sqrtx)/(x-1) - 4/(x+sqrtx)) : (3/(sqrtx +1) - 1/(sqrtx -1))`

`= [sqrtx/((sqrtx -1)(sqrtx +1)) - 4/(sqrtx (sqrtx +1))] : (3(sqrtx -1) - (sqrtx +1))/((sqrtx -1)(sqrtx +1))`

`=  (x-4(sqrtx -1))/(sqrtx (sqrtx -1)(sqrtx +1)) : (3sqrtx -3 - sqrtx -1)/((sqrtx -1)(sqrtx +1))`

`= (x - 4sqrtx +4)/(sqrtx (x-1)) : (2sqrtx -4)/(x-1)`

`= (sqrtx -2)^2/(sqrtx (x-1)) : (2(sqrtx -2))/(x-1)`

`= (sqrtx -2)^2/(sqrtx (x-1)) . (x-1)/(2(sqrtx -2))`

`= (sqrtx -2)/(2sqrtx)`