3a+2b+c=5 ①
2a+b-3c=1 ②
①－②×2:-a+7c=3,a=7c-3
b=1-2a+3c=1-2(7c-3)+3c=-11c+7
因a≥0,b≥0,c≥0
7c-3≥0,c≥3/7
-11c+7≥0,c≤7/11
3/7≤c≤7/11
m=3a+b-7c=3(7c-3)+(-11c+7)-7c=3c-2
當(dāng)c=3/7時(shí),m有最小值：x=3×﹙3/7﹚-2=-5/7
當(dāng)c=7/11時(shí),m有最大值：y=3×﹙7/11﹚-2=-1/11