$\"\"$

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Discuss the derivation:

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Consider $\"\"$ .

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Sum formula of tangent function $\"\"$ .

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Here $\"\"$ .

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$\"\"$ .

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$\"\"$

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$\"\"$

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Divide numerator and denominator by $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$ $\"\"$ .

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$\"\"$ \ \

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Substitute $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Apply reciprocal identity: $\"\"$ .

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$\"\"$ .

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$\"\"$ .

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$\"\"$

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$\"\"$ .