Question 1152698
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A(x)=x(400-x), where x is the width of the pen. Is A(x) a quadratic function? 
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Obviously yes.



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What is the maximum area of the pen? 
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roots 0 and 400, exact middle being 200 for x.
Maximum area {{{200*(400-200)=200*200=highlight_green(40000)}}}



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What are the dimensions of the maximum pen?
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Look again; you can already see this.