y=（x-40）[500-（x-50）×10]

=（x-40）（1000-10x）

=-10x2+1400x-40000

=-10(x-70)2+9000．

=9000-20 (x-70)

When the subtraction item is zero, the profit is maximum

20 (x-70)=0

x=70

This is a function quadrant problem. What we look at is judgment, analysis, and reasoning.