B1(0,b),M(c,d),B2(0.-b)B1M方程:(y-b)/x=(d-b)/c.交x轴于P(-bc/(d-b).0)B2M方程:(y+b)/x=(d+b)/c.交x轴于P(bc/(d+b).0)|Om|·|On|=|-b²c²/(d²-b²)|.注意c²/a²+d²/b²=1.c²=(b²-d²)a²/b².代人计算得|Om|·|On|=a²