参考一下:已知x.y.z为三个非负有理数,且满足3x+2y+z=5,x+y-z=2,若s=2x+y-z,求s的最大值与最小值.解:3x+2y+z=5①2x+y-3z=1②①-②*2得7z-x=3∴z=(x+3)/7
