当k=( )时多项式-x^2+2kxy-4y^2+xy-5x-1不含xy项

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当k=( )时多项式-x^2+2kxy-4y^2+xy-5x-1不含xy项
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当k=( )时多项式-x^2+2kxy-4y^2+xy-5x-1不含xy项
当k=( )时多项式-x^2+2kxy-4y^2+xy-5x-1不含xy项

当k=( )时多项式-x^2+2kxy-4y^2+xy-5x-1不含xy项
-x^2+2kxy-4y^2+xy-5x-1
=-x^2+(2k+1)xy-4y^2-5x-1
要不含xy项,必须:
2k+1=0
k=-1/2

K=-2分之1

不含xy项即 2kxy+xy=0
即 2k+1=0
则 k=-0.5