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Next use Heron’s formula to compute the area:
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\[
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A = \sqrt{s(s - a)(s - b)(s - c)} = \sqrt{21(21 - 13)(21 - 14)(21 - 15)} = \sqrt{21 \times 8 \times 7 \times 6}
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\]
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\[
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= \sqrt{21 \times 8 \times 7 \times 6} = \sqrt{7056}
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\]
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Note that \( 7056 = 84^2 \) so:
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\[
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A = 84 \ \text{cm}^2
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\]
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Now compute the inradius:
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