请教bisakiki：这道题做过没？

timetoleave

7.  One kilogram of a certain coffee blend consists of x kilogram of type I coffee and y kilogram of type II coffee.  The cost of the blend is C dollars per kilogram, where C = 6.5x + 8.5y.  Is x < 0.8 ?

 

(1) y > 0.15

 

(2) C 》＝7.30

答案是B？咋推的呢？

谢谢！！！

bisakiki

哈。。。我来了！

MM是不是忽略了个条件：x+y=1

1）y>0.15这个没什么用处，X可以等于0.75-0.8之间的任意数，所以不一定小于0.8

2)把y=1-x带入：C=6.5x + 8.5(1-x)=-2x+8.5>=7.3

then移项,-2x>=-1.2 x<=0.6 这样就肯定< 0.8了