椭圆x^2/a^2+y^2/b^2=1(a>b>0)与x轴交于点A1(-a,0),A2(a,0),与y轴交于B1(0,-b),B2(0,b),则A1A2是椭圆的长轴，B1B2是椭圆的短轴。焦距为2√(a^2-b^2).
