三元一次方程简单应用{x+2y+z=72x-y+3z=73x+y+2z=18}组：{x+2y+z=7 ①2x-y+3z=7 ②3x+y+2z=18 ③ }①+②×2得：5x+7z=21 ④②+③得：x+z=5 ⑤联立④、⑤得：{5x+7z=21x+z=5}利用二元一次方程解法解得：{x=7,z=-2}把x...