so.. if you notice the picture below.. .the length is that much
thus [tex]\bf A=lw\qquad \cfrac{1800-3w}{2}=l\implies 900-\cfrac{3w}{2}=l
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A=\left( 900-\cfrac{3w}{2} \right)w\implies A=900w-\cfrac{3w^2}{2}\implies A=900w-\cfrac{3}{2}w^2[/tex]
take the derivative of A, zero it out, and then check the critica points for any maxima by using the first-derivative test
