x2>x1>-1
f(x2)-f(x1)=ax2^2+(x2-2)/(x2+1)-ax1^2-(x1-2)/(x1+1)
=a(x2-x1)(x2+x1)-(x2-x1)/[(x2+1)(x1+1)]
=(x2-x1)[a(x2+x1)-3/(x2+1)(x1+2)]
>0
函数f(x)在(-1,+∞)上为增函数,
f(0)=-20(a>1)
方程f(x)=0在（0,1）内必有实根