（x+2)/(x-1)=(x-1+3)/(x-1)=1+3/(x-1)
因为（x+2)/(x-1)为正整数,所以1+3/(x-1)为 正整数,所以3/(x-1)为非负整数,
x-1=1,3,-1,-3
x=2,4,0,-2,
其中正整数有2,4,所以x=2,或4.(x-1+3)/(x-1)=[(x-1)+3]/(x-1)=（x-1)/(x-1)+3/(x-1)=1+3/(x-1)