椭圆x的平方除以a的平方+y的平方除以b的平方=1(a>b>0)与直线x+y-1=0交于M,N两点 且以MN为直径的圆过

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  • (1)

    x^2/a^2+y^2/b^2=1

    b^2x^2+a^2y^2=a^2b^2

    x+y-1=0

    y=-x+1

    将y=-x+1,代入b^2x^2+a^2y^2=a^2b^2,

    b^2x^2+a^2(-x+1)^2=a^2b^2,

    即(b^2+a^2)x^2-2a^2x+a^2-a^2b^2=0,

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

    x1*x2=(a^2-a^2b^2)/(b^2+a^2),

    ∵OP⊥OQ,

    ∴x1*x2+y1*y2=0,

    即x1*x2+(x1-1)*(x2-1)=0

    ∴2x1*x2-(x1+x2)+1=0,

    ∴2a^2-2a^2b^2-2a^2+b^2+a^2=0,

    得,

    b^2+a^2=2a^2b^2,

    即1/a^2+1/b^2=2,

    (2)

    e^2=1-a^2/b^2=1+1/(1-2a^2),

    ∵1/2≤e^2≤3/4,

    ∴3≤2a^2≤5,

    ∴长轴范围是√6≤2a ≤√10