-2kxy-3xy+(1/2)xy-x+8
=[-2k-(5/2)]xy-x+8
-2kxy-3xy+(1/2)xy-x+8中不含xy项
则xy项前的系数为0
-2k-(5/2)=0
解得k=-5/4
所以k=-5/4是-2kxy-3xy+(1/2)xy-x+8中不含xy项