题目不全 ,但还是找到了答案不知道是不是你要的
设p(x0,y0),M(x1,y1),N(x2,y2),(1)X0^2/a^2+y0^2/b^2=1,(2)X1^2/a^2+y1^2/b^2=1,(3)X2^2/a^2+y2^2/b^2=1.(2)-(1)得（X1^2-x0^2)/a^2+（y1^2-y0^2)/b^2=1,整理得PM的斜率K1=(y0-y1)/(x0-x1)=b^2(x0+x1)/a^2(y0+y1),同理PN的斜率K2=(y0-y2)/(x0-x2)=b^2(x0+x2)/a^2(y0+y2),K1*K2=[b^4(x0+x1)(x0+x2)]/[a^4(y0+y2)(y0+y1)]=|1\4|,M、N是椭圆上关于原点对称的两点,x1=-x2,y1=-y2.
b^2/a^2=3/4,(a^2-c^2)/a^2=3/4,e=√3/2.