三角形ABC的三边a>b>0成等差数列
|AB|+|BC|=c+a=2b=(1-(-1))
b=1
c+a=2
B的轨迹为以A.C两点为焦点的椭圆
(x^2/a'^2)+(y^2/b'^2)=1(a'>b'>0)
c'=b/2
a'=(c+a)/2=1
b'^2=3/4
x^2+(y^2/(3/4))=1(y≠0)