已知椭圆C:x2/a2+y2/b2=1(a>b>0)的离心率e=根6/3过点A(0,-b)和B(a,0)的直线与原点的距

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  • ∵“过点A(0,-b)和B(a,0)的直线与原点的距离为根3/2”,∴a·b = (√3/2)·√(a^2 + b^2),∵e = √6/3 = c/a ,联合解得:a = √3 ,b = 1 ,c = √2,椭圆方程:(x^2/3) + y^2 = 1.

    假设C(x1,y1) ,D(x2,y2),

    要使“以CD为直径的圆过点E”,则CE⊥DE,∴CE、DE斜率乘积 = -1

    即:[y1/(x1 + 1)]·[y2/(x2 + 1)] = -1 ,即(y1·y2)/[(x1·x2) + (x1 + x2) + 1] = -1.T式

    将y = kx + 2代入椭圆方程得:

    x^2 + 3(kx + 2)^2 = 3

    或3y^2 + [(y - 2)^2/(k^2)] = 3

    分别整理得:(1 + 3k^2)x^2 + 12kx + 9 = 0

    以及:[3 + (1/k^2)]y^2 -(4y/k^2) + [(4/k^2) - 3] = 0

    根据韦达定理:x1x2 = 9/(1 + 3k^2) ,x1 + x2 = -12k/(1 + 3k^2),y1y2 = (4 - 3k^2)/(1 + 3k^2)

    ∴(x1·x2) + (x1 + x2) + 1 = (10 - 12k + 3k^2)/(1 + 3k^2),代入T式:

    (4 - 3k^2)/(10 - 12k + 3k^2) = -1

    ∴6k^2 - 12k + 6 = 0,解得k = 1,直线为:y = x + 2

    因此,存在k = 1使得 以CD为直径的圆过点E