nesstang

我来啦！
gross profit=revenue-cost
revenue=12x,
gross profit=-2(x-5)^2+10,
so,cost=12x+2(x-5)^2-10=2x^2-8x+40

单个的bowl（千）的cost=总cost/x=2x-8+40/x
3千个（x=3）时对应的单个bowl-4千个（x=4）时对应的单个bowl
=2*3-8+40/3-(2*4-8+40/4)

这下清晰了吧~