f(x)=（ax+1）/(x+2) = (a(x+2)-2a+1)/(x+2) = a + (1-2a)/(x+2) f(x)=（ax+1）/(x+2)在区间(－2，＋∞)上单调递增 <=> (1-2a)/(x+2) 在区间(－2，＋∞)上单调递增 <=> 1-2a < 0 <=> a > 1/2实数a的取值范围是 (1/2, ＋∞)
