1.
x=2y-5 x-2y=-5
3x-2y=6
相减：2x=11
x=11/2
y=-[6-3*(11/2)]/2
=21/4
2.
x+3y=4
(x/4)+(y/2)=0
(4-3y)/4+(y/2)=0
(4-3y+2y)=0
y=4
x=4-3*4
=-8
3.
2000x+1999y=3997
1999x+2000y=4001
相加：3999(x+y)=7998
x+y=2
2000x+1999*(2-x)=3997
x=3997-1999*2
=-1
y=-3