问题 解答题

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.

答案

x+y+z=30        ①
3x+y-z=50        ②

由①+②得  4x+2y=80,y=40-2x   ③,

把③代入①得  z=x-10   ④,

所以:M=5x+4(40-2x)+2(x-10)=-x+140,即x=140-M   ⑤,

分别将⑤代入③④,

x=140-M≥0
y=2M-240≥0
z=130-M≥0

解得

M≤140
M≥120
M≤130

所以120≤M≤130.

答:M的取值范围为120≤M≤130.

问答题 简答题
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     请认真阅读下列短文,并根据所读内容在文章后表格中的空格里填入最恰当的单词。注意:每空格

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     Do you raise salmon (鲑鱼) in your classroom at school? Do you ever have class while floating down a

river in a canoe? Do you ever throw your school lunch leftovers into a compost (混合肥料) bin when you

are finished? They are all part of a regular school day at Barnard Environmental Studies Magnet School in

New Haven, Connecticut. Known as a "green" school, Barnard attracts students from all over the region who

are interested in environmental studies.

     "We do Earth Day all the year round," said Marjorie Drucker, Barnard's Magnet Resource teacher. With

the school's special classes, its 325 students study the environment all day long. "Being green means that

everything in the building is designed with the environment in mind," Drucker told Scholastic News. For

example, the lights go off when people leave the room, and special windows provide "passive light", cutting

down on the need for electric lighting. When a window is open, the air-conditioning automatically turns off.

     Conserving energy is not all that students do to help the environment. While learning about the dying

salmon population in the Connecticut rivers, students also do something to help out-they raise more salmon.

In the classrooms there are fish tanks containing salmon eggs. The eggs are kept in water at two degrees

Celsius. After the eggs hatch, students observe the baby salmon through all the stages of development, and

then release the adult salmon into Connecticut rivers.

     Composting is another part of taking care of the environment at Barnard Students deal with their lunches

by throwing the remaining food into a compost bin in a greenhouse. The compost is used to help grow plants.

     At Barnard, almost everything has to do with the environment. Students sing songs about the environment;

their artworks have something to do with the environment; even their math classes are focused on protecting

the environment.

     Barnard offers chances that students can't get in a regular school. The school's goal is to teach children to

be good environmental role models. They also learn to live in a different way. To attend the school, students

have to apply and express their interest in protecting the environment. "Students come here because they care

about the environment," said Drucker.