1.x+y=3a 2.y+z=5a →1-2=x-z=-2a→ 4.x=z-2a3.z+x=4a →代入3可得z=3a 依次代入可得x=a,y=2a代数式(x^2)-2y+3z的值为-10可得：(x^2)-2y+3z=-10a^2-4a+9a=10a^2-5a-10=0(a-2.5)^2=16.25a=±√16.25+2.5...