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The poster design should be top and bottom margins are each $\"\"$ cm and the side margins are each $\"\"$ cm.

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$\"\"$

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Consider $\"\"$ and $\"\"$ are the length and width of the printed material on the poster.

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Therefore $\"\"$ and $\"\"$ are the length and width of the poster.

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The area of the printed material on the poster is $\"\"$ .

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But area of the printed material is $\"\"$ cm².

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$\"\"$

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The area of the total poster is $\"\"$ .

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$\"\"$

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Apply derivative on each side with respect to $\"\"$ .

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$\"\"$

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The area of the poster is maximum when its first derivative becomes zero.

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Equate $\"\"$ to zero.

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$\"\"$

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Therefore the total length of the poster is $\"\"$ cm.

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Substitute $\"\"$ in $\"\"$ .

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$\"\"$

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Therefore the total width of the poster is $\"\"$ cm.

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The total length of the poster is $\"\"$ cm.

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The total width of the poster is $\"\"$ cm.