x轴和y轴
(0,0)
[-a,a]
[-b,b]
A1(-a,0),A2(a,0),
B1(0,-b),B2(0,b)
A1(0,-a),A2(0,a),
B1(-b,0),B2(b,0)
2b
2a
F1(-c,0),F2(c,0)
F1(0,-c),F2(0,c)
2c