$\"\"$

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The trigonometric function is $\"\"$ .

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The equation is $\"\"$ .

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$\"\"$

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From Pythagorean triangle $\"\"$ .

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$\"\"$

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Pythagorean theorem $\"\"$ .

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$\"\"$

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$\"\"$

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Substitute corresponding values in the above equation.

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$\"\"$

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$\"\"$

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Substitute corresponding values in the above equation.

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$\"\"$ .

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$\"\"$

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Find $\"\"$ .

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Double-angle formula: $\"\"$ .

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Substitute $\"\"$ and $\"\"$ in double-angle formula.

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$\"\"$

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$\"\"$

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Therefore, $\"\"$ .

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$\"\"$

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$\"\"$ .