Answer:
-$0.07846 per belt
Step-by-step explanation:
The average cost per belt is ...
[tex]c(x)=\dfrac{C(x)}{x}=\dfrac{751+12x-0.067x^2}{x}=751x^{-1}+12-0.067x[/tex]
Then the rate of change of average cost is ...
[tex]c'(x)=-751x^{-2}-0.134\\\\c'(256)=\dfrac{-751}{256^2}-0.067\approx -0.07846[/tex]
The rate at which average cost is changing is about -0.078 dollars per belt.
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Note that the cost of producing 256 belts is -$567.91, so their average cost is about -$2.22 per belt.