$\"\"$

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To prove the identity draw a right triangle :

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$\"\"$

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Pythagorean theorem : $\"\"$ .

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$\"\"$

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$\"\"$

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Divide each side by $\"\"$ .

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$\"\"$

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From the diagram :

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

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Substitute the values in equation (1).

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$\"\"$

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$\"\"$

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$\"\"$ .