求满足方程xy=20-3x+y的所有整数对(xy)
来源:百度知道 编辑:UC知道 时间:2024/06/01 08:07:18
正确的解法如下:
解:将已知变形为:
xy+3x-y=20
x(y+3)-(y+3)=17
(x-1)(y+3)=17=1×17=-1×(-17)
所以有四种情形:
①
(x-1)=1
(y+3)=17
得:x=2,y=14;
②
(x-1)=17
(y+3)=1
得:x=18,y=-2;
③
(x-1)=-1
(y+3)=-17
得:x=0,y=-20;
④
(x-1)=-17
(y+3)=-1
得:x=-16,y=-4;
综上,所有整数对(xy)为:
(2,14)(18,-2)(0,-20)(-16,-4)
3X+Y=K+1①
X+2Y=3 ②
用①*2-②得:
5X=2K-1
X=(2K-1)/5
代入②得:
[(2K-1)/5]+2Y=3
2Y=3-[(2K-1)/5]=(16-2K)/5
Y=(8-K)/5
X-Y
=[(2K-1)/5]-[(8-K)/5]
=(3K-9)/5
当K=2时:X-Y=-3/5
当K=4时:X-Y=3/5
所以X-Y的范围是:-3/5<X-Y<3/5
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