Question

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Answer 1

Apply the Pythagorean theorem:
${a}^{2} + {b}^{2} = c ^{2}$
${c}^{2} = {20}^{2} +{21}^{2} \\ {c }^{2} = 400 + 441 \\ {c}^{2} = 841 \\ \sqrt{ {c}^{2} } = \sqrt{841} \\ c = 29$