|F1F2|=2c>0,设F1,F2的坐标分别为F1(-c,0),F2(c,0).
C上任意一点P的坐标为(x,y):
|CF1|^2 + |CF2|^2 = 2a^2
|CF1|^2 = (x+c)^2 + y^2
|CF2|^2 = (x-c)^2 + y^2
(x+c)^2 + y^2 + (x-c)^2 + y^2 = 2a^2
简化得:x^2 + y^2 = a^2 - c^2
a < c时,a^2 - c^2 < 0,曲线C不存在
a = c时,a^2 - c^2 = 0,曲线C是原点
a > c时,a^2 - c^2 > 0,曲线C是以原点为圆心,半径为sqrt(a^2-c^2)的圆 (sqrt为平方根).