$\"\"$

\

The cofunction identity is $\"\"$ .

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Consider left hand side identity : $\"\"$ .

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Apply quotient identity : $\"\"$ .

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$\"\"$

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Apply difference identities : $\"\"$

\

$\"\"$ .

\

$\"\"$

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Apply quotient identity : $\"\"$ .

\

$\"\"$

\

= Right hand side identity.

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$\"\"$

\

$\"\"$ .