Đáp án:
`2a)Q=(1/(sqrta-1)-1/sqrta):((sqrta+1)/(sqrta-2)-(sqrta+2)/(sqrta-1))(a>0,a ne 1,a ne 4)`
`Q=(sqrta/(sqrta(sqrta-1))-(sqrta-1)/(sqrta(sqrta-1))):(((sqrta+1)(sqrta-1))/((sqrta-1)(sqrta-2))-((sqrta+2)(sqrta-2))/((sqrta-1)(sqrta-2)))`
`Q=(sqrta-sqrta+1)/(sqrta(sqrta-1)):(a-1-a+4)/((sqrta-1)(sqrta-2))`
`Q=1/(sqrta(sqrta-1)):3/((sqrta-1)(sqrta-2))`
`Q=1/(sqrta(sqrta-1))*((sqrta-1)(sqrta-2))/3`
`Q=(sqrta-2)/(3sqrta)`
`b)Q>0`
`<=>(sqrta-2)/(3sqrta)>0`
Mà `3sqrta>0AAa>0`
`<=>sqrta-2>0`
`<=>sqrta>2`
`<=>a>4`
Vậy với `a>4` thì `Q` dương.