y=2x/(x+1)^2
y'=2[(x+1)^2-2(x+1)x]/(x+1)^4=2[x+1-2x]/(x+1)^3=2(1-x)/(x+1)^3
y"=2[-(x+1)^3-3(x+1)^2(1-x)]/(x+1)^6=2[-x-1-3+3x]/(x+1)^4=4(x-2)/(x+1)^4
由y"=0,得x=2,
y(2)=4/9
当x>2时,y">0, 为凹区间；
当-1