当k为何值时,x^2-2kxy-y^2+1/2xy-1中不含xy项

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当k为何值时,x^2-2kxy-y^2+1/2xy-1中不含xy项

当k为何值时,x^2-2kxy-y^2+1/2xy-1中不含xy项
当k为何值时,x^2-2kxy-y^2+1/2xy-1中不含xy项

当k为何值时,x^2-2kxy-y^2+1/2xy-1中不含xy项
x^2-2kxy-y^2+1/2xy-1
=x^2+(1/2-2k)xy-y^2-1
不含xy项,xy项系数=0
1/2-2k=0
k=1/4