椭圆基础问题过椭圆X^2/a^2+y^2/b^2=1一个焦点F作两条相互垂直的直线L1,L2,L1交椭圆于AB两点,L2
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设L1:y=k(x-c),其中c=√(a^2-b^2),

代入椭圆方程得b^2x^2+a^2k^2(x-c)^2=a^2b^2,

(b^2+a^2k^2)x^2-2a^2ck^2x+a^2(c^2k^2-b^2)=0,

设A(x1,y1),B(x2,y2),则

x1+x2=2a^2ck^2/(b^2+a^2k^2),

x1x2=a^2(c^2k^2-b^2)/(b^2+a^2k^2),

|x1-x2|=√[(x1+x2)^2-4x1x2]

=2ab^2√(1+k^2)/(b^2+a^2k^2),

|FA|=|x1-c|√(1+k^2),

|FB|=|x2-c|√(1+k^2),

∴|FA|*|FB|=|(x1-c)(x2-c)|(1+k^2)

=|[x1x2-c(x1+x2)+c^2]|(1+k^2)

=|[a^2(c^2k^2-b^2)-2a^2c^2k^2+c^2(b^2+a^2k^2)]|(1+k^2)/(b^2+a^2k^2)

=b^4(1+k^2)/(b^2+a^2k^2),

FC*FD=b^4(1+1/k^2)/(b^2+a^2/k^2)=b^4(1+k^2)/(a^2+b^2k^2)≠FA*FB,

(1)1/FA + 1/FB

=(FA+FB)/(FA*FB)

=(x1-x2)√(1+k^2)/(-|FA|*|FB|)

=2a/[b^2*√(1+k^2)],

∵L1⊥L2,

∴以-1/k代k,得

1/FC + 1/FD

=2a/[b^2*√(1+1/k^2)]

≠1/FA + 1/FB,

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