$\"\"$

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From exercise (28) in section 9.2.

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The differential equation is $\"\"$ .

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$\"\"$

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Apply integral on each side.

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$\"\"$

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$\"\"$

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Exponentiate each side.

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .

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$\"\"$

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The initial temperature is $\"\"$ , when $\"\"$ .

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

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .

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

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$\"\"$ .

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$\"\"$

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$\"\"$ .