Hướng dẫn trả lời:
3.
`B = (1/{sqrt{x} + 3} + 1/{sqrt{x} - 3}).{4sqrt{x} + 12}/sqrt{x}.` (ĐKXĐ: x > 0; x $\neq$ 9)
`B = ({sqrt{x} - 3}/{(sqrt{x} + 3).(sqrt{x} - 3)} + {sqrt{x} + 3}/{(sqrt{x} + 3).(sqrt{x} - 3)}).{4.(sqrt{x} + 3)}/sqrt{x}.`
`B = [{(sqrt{x} - 3) + (sqrt{x} + 3)}/{(sqrt{x} + 3).(sqrt{x} - 3)}].{4.(sqrt{x} + 3)}/sqrt{x}.`
`B = ({sqrt{x} - 3 + sqrt{x} + 3}/{(sqrt{x} + 3).(sqrt{x} - 3)}).{4.(sqrt{x} + 3)}/sqrt{x}.`
`B = {2sqrt{x}}/{(sqrt{x} + 3).(sqrt{x} - 3)}.{4.(sqrt{x} + 3)}/sqrt{x}.`
`B = 8/{sqrt{x} - 3}.`
4.
`{x + 1 - 2sqrtx}/{sqrtx - 1} + {x + sqrtx}/{sqrtx + 1}` (ĐKXĐ: x ≥ 0; x $\neq$ 1).
`= {(sqrtx)^2 - 2.sqrtx.1 + 1^2}/{sqrtx - 1} + {sqrtx.(sqrtx + 1)}/{sqrtx + 1}`
`= {(sqrtx - 1)^2}/{sqrtx - 1} + {sqrtx.(sqrtx + 1)}/{sqrtx + 1}.`
`= sqrtx - 1 + sqrtx.`
`= 2sqrtx - 1.`