$\"\"$

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c) $\"italic$

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Apply half-angle formula: $\"italic$ .

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$\"table$ (Substitute $\"begin$ )

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$\"table$

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$\"table$

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$\"table$

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$\"table$ (Multiply numoretor and denominator by $\"begin$ )

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$\"equal$

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$\"\"$

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The exact value of $\"italic$ $\"equal$ .