$\"\"$

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The identity is $\"\"$ .

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Consider left hand side identiy $\"\"$ .

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Quotient identity : $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Pythagorean identity : $\"\"$ .

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$\"\"$

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= Right hand side of the identity.

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$\"\"$

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$\"\"$ .