\"\"

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The side lengths of a right triangle are \"\" ; \"c.

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Apply Pythagorean theorem: \"a

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\"\"              (Substitute 15 for a and 20 for b)

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\"\"            (Evaluate powers: \"\")\"\"

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\"\"                       (Add: \"400)

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\"\"                (Apply square root on each side)

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\"\"                 (Write \"\")

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\"\"                           (Cancel sqaure and root)\"\"

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The unknown length of \"\".