$\"\"$

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The expression is $\"\"$ .

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$\"\"$ .

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Let $\"\"$ in the interval $\"\"$ .

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Let $\"\"$ in the interval $\"\"$ .

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$\"\"$ .

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Sum formula of cosine function: $\"\"$ .

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$\"\"$ .

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Find $\"\"$ .

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$\"\"$

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$\"\"$

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From Pythagorean theorem,

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The square of the hypotenuse is equal to sum the squares of the other two sides.

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .

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Find $\"\"$ .

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Consider $\"\"$ .

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$\"\"$ .

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From Pythagorean theorem,

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The square of the hypotenuse is equal to sum the squares of the other two sides.

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$\"\"$

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$\"\"$ .

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$\"\"$ .

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Substitute $\"\"$ , $\"\"$ , $\"\"$ and $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ , where $\"\"$ and $\"\"$ .

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$\"\"$ , where $\"\"$ and $\"\"$ .