f(x)=(ax+1)/(x+2)=(a(x+2)+(1-2a))/(x+2)= a +(1-2a))/(x+2)
a>1/2 ,1-2a < 0 在区间（-2,+00)上,
(x+2)单调递增,1/（x+2)单调递减,(1-2a))/(x+2) 单调递增,
∴ 函数f(x)=(ax+1)/(x+2))= a +(1-2a))/(x+2)
在区间（-2,+00)上单调递增