若x+y+z=30,3x+y-z=50,x、y、z皆为非负数,求M=5x+4y+2z的取值范围.
问题描述:
若x+y+z=30,3x+y-z=50,x、y、z皆为非负数,求M=5x+4y+2z的取值范围.
答
x+y+z=30 ①3x+y−z=50 ②,由①+②得 4x+2y=80,y=40-2x ③,把③代入①得 z=x-10&nb...