The expression is .

The expression is in the form .

Let in the interval .

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From Pythagorean theorem,

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

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Let  in the interval .

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From Pythagorean theorem,

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

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Substitute and in trigonometric expression.

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Sum formula of cosine function : .

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Substitute these values in above expression.

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The algebraic expression for is , where and .