$\"\"$

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The model is $\"\"$ , where $\"\"$ is sales in and time $\"\"$ is in months.

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(a). Graph the function $\"\"$ .

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$\"\"$

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Observe the graph:

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The area of the region above the $\"\"$ -axis equals the area of the region below the $\"\"$ -axis.Thus, the area value is zero.

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$\"\"$ .

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$\"\"$

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Average value of the function $\"\"$ on $\"\"$ is defined as $\"\"$ .

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In this case $\"\"$ and $\"\"$ .

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Average value is $\"\"$

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Substitute $\"\"$ .

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$\"\"$

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$\"\"$ .

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The average value of $\"\"$ is $\"\"$ .

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$\"\"$

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(b) Graph the functions $\"\"$ and $\"\"$ in a single viewing window.

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$\"\"$

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Observe the graph:

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The average value of $\"\"$ appears to be $\"\"$ .

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The best line drawn through a set of data is called trend line.

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The function $\"\"$ is the trend line of $\"\"$ .

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$\"\"$

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(a). Graph of the function $\"\"$ .

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$\"\"$

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The area above the $\"\"$ -axis equals the area below the $\"\"$ -axis.Thus, the average value is zero.

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(b). Graph of the functions $\"\"$ and $\"\"$ in a single viewing window.

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$\"\"$

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The average value of $\"\"$ appears to be $\"\"$ .