a+b=1,ab=-3.求a^2+ab和2a^3b+2ab^3的积

2个回答

  • a+b=1

    两边平方;

    (a+b)^2=1*1

    ∴a^2+2ab+b^2=1

    ∴a^2+b^2=1-2ab=1+2*3=7

    2a^3b+2ab^3=2ab(a^2+b^2)=2*(-3)*7=-42;

    a+b=1,

    两边*a

    aa+ab=a;

    a+b=1,

    b=1-a代入ab=-3;

    a=(1+√13)/6;或者a=(1-√13)/6

    (a^2+ab)*(2a^3b+2ab^3)

    =(-42)*a

    =-42a

    (a^2+ab)*(2a^3b+2ab^3)

    =-42*(1+√13)/6

    =-7-7√13;

    或者(a^2+ab)*(2a^3b+2ab^3)

    =-42*(1-√13)/6

    =-7+7√13;

    ab=-3

    ∴2a^3b+2ab^3