$\"\"$

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Observe the figure, it consists two rectangles.

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The length of small rectangle $\"\"$ and its width $\"\"$ .

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The area of small rectangle is $\"\"$ .

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The values of $\"\"$ are substitute in the equation $\"\"$ . \ \

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$\"\"$

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$\"\"$ .

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$\"\"$

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The length of big rectangle $\"\"$ and its width $\"\"$ .

\

The area of big rectangle is $\"\"$ .

\

The values of $\"\"$ are substitute in the equation $\"\"$ . \ \

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$\"\"$

\

$\"\"$ .

\

$\"\"$

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The area of the shaded region is $\"\"$ .

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The values of $\"\"$ are substitute in the equation $\"\"$ .

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$\"\"$

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$\"\"$ . $\"\"$

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The area of the shaded region is $\"\"$ .