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.

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...