当x=2,y=-4时,
代数式ax3+1/2by+5
=8a-2b+5=1997,
∴8a-2b=1992,
4a-b=996
当x=1,y=-1/2时,-------------------只有Y求不出,添加X=1
代数式ax3+1/2by+5
=a-1/4b+5
=1/4(4a-b)+5
=249+5
=254
当x=2,y=-4时,
代数式ax3+1/2by+5
=8a-2b+5=1997,
∴8a-2b=1992,
4a-b=996
当x=1,y=-1/2时,-------------------只有Y求不出,添加X=1
代数式ax3+1/2by+5
=a-1/4b+5
=1/4(4a-b)+5
=249+5
=254