Đáp án:
`a)P=(x+3)/(sqrtx+1)=(x-1+4)/(sqrtx+1)=sqrtx-1+4/(sqrtx+1)=sqrtx+1+4/(sqrtx+1)-2>=4-2=2` $\\$ Dấu "=" xảy ra khi `sqrtx+1=2<=>x=1` $\\$ `b)x>=0=>sqrtx+2>=2>0,x>=0=>P>=0` Dấu "=" xảy ra khi `x=0` $\\$ `c)P=(x+8)/(sqrtx+1)=(x-1+9)/(sqrtx+1)=sqrtx-1+9/(sqrtx+1)=sqrtx+1+9/(sqrtx+1)-2>=6-2=4` $\\$ Dấu "=" xảy ra khi `sqrtx+1=3<=>x=4` $\\$ `d)P=(4x+5)/(sqrtx+1)=(4x-4+9)/(sqrtx+1)=4(sqrtx-1)+9/(sqrtx+1)=4(sqrtx+1)+9/(sqrtx+1)-4>=12-4=8` $\\$ Dấu "=" xảy ra khi `sqrtx+1=3/2<=>x=1/4`