x+y+z=12 x-y+z=2 x-z+y+a=90 x+y+a+z=100,求x y z a
问题描述：
x+y+z=12 x-y+z=2 x-z+y+a=90 x+y+a+z=100,求x y z a
答
x+y+z=12① x-y+z=2② x-z+y+a=90③ x+y+a+z=100④
①+②得2x+2z=14 x+z=7⑤
②+④得2x+2z+a=102⑥
⑥-⑤*2得a=88⑦
②+③得2x+a=92⑧
把⑦代入⑧中得2x=4 x=2⑨
把⑨代入⑤中得z=5⑩
把⑨⑩代入①得y=5
∴x=2,y=5,z=5,a=88
(打得好累哦)