`a)B=(sqrtx-(3x+1)/(x+3sqrtx))*(sqrtx+3)/(sqrtx-1)(0<x ne 1)`
`B=((xsqrtx+3x-3-1)/(x+3sqrtx))*(sqrtx+3)/(sqrtx-1)`
`B=((xsqrtx-1)/(sqrtx(sqrtx+3))*(sqrtx+3)/(sqrtx-1)`
`B=((sqrtx-1)(x+sqrtx+1))/(sqrtx(sqrtx+3))*(sqrtx+3)/(sqrtx-1)`
`B=(x+sqrtx+1)/sqrtx`
`b)P=A.B`
`P=(x+sqrtx+1)/(sqrtx-1)` không tìm được GTNN.
Chuẩn hóa:`x>1`
`P=(x-2sqrtx+1+3sqrtx-3+3)/(sqrtx-1)`
`P=((sqrtx-1)^2+3(sqrtx-1)+3)/(sqrtx-1)`
`P=sqrtx-1+3+3/(sqrtx-1)`
`P>=2sqrt3+3`(cosi)
Dấu "=" `<=>(sqrtx-1)=3/(sqrtx-1)<=>sqrtx-1=sqrt3<=>x=(sqrt3+1)^2=4+2sqrt3`