3x+2y=23
2x+y+z=17 4x+2y+2z=34
x+y+2z=15
(4x+2y+2z)-(x+y+2z)=34-15
3x+y=19
(3x+2y)-(3x+y)=23-19
y=4
代入式子可得：x=5 y=4 z=34x+2y+2z=34这一步是怎么来的2x+y+z=17
(2x+y+z)*2=17*2
4x+2y+2z=34