y=ax+2带入x2/3a2+y2/a2=1
得到
x^2/3a^2+(ax+2)^2/a^2=1
x^2+3(a^2*x^2+4ax+4)=3a^2
(1+3a^2)x^2+12ax+12-3a^2=0
(12a)^2-4(1+3a^2)(12-3a^2)>0才会有两个交点
从而推出a的取值范围 -1