椭圆x^2/3+y^2=1与过点p(0,2)直线L交AB,求三角形AOB(o为原点)面积最大时L方程

4个回答

  • a=√3,b=1,c=√2,

    设直线方程为:y=kx+2,

    kx-y+2=0,

    设A(x1,y1),B(x2,y2)

    O至直线距离d=2/√(1+k^2),

    x^2/3+(kx+2)^2=1,

    (1+3k^2)x^2+12kx+9=0,

    根据韦达定理,

    x1+x2=-12k/(1+3k^2),

    x1x2=9/(1+3k^2),

    根据弦长公式,

    |AB|=√(1+k^2)(x1-x2)^2

    =√(1+k^2)[(x1+x2)^2-4x1x2]

    =√(1+k^2)[ 144k^2/(1+3k^2)^2-36/(1+3k^2)]

    =[6/(1+3k^2)]√[(1+k^2)(k^2-1)],

    S△OAB=d*|AB|/2=[2/√(1+k^2)]*[6/(1+3k^2)]√[(1+k^2)(k^2-1)]/2

    =6√(k^2-1)/(1+3k^2),

    设S=S△OAB,

    t=k^2,

    S=6√(t-1)/(1+3t),(1)

    S^2=36(t-1)/(1+3t)^2,

    设A=S^2,

    9At^2+6t(A-6)+36+A=0,

    当△>=0时,二次方程有实根,

    36(A-6)^2-4*9A(36+A)>=0,

    A