<c = 27°
c² = a² + b² - 2abcosc (law of cosines)
13² = 17² + b² - 2 × 17 × bcos27°
169 = 289 + b² - 34b × 0.89
b² - 30.29b + 120 = 0
b1 ≈ 4.69 b2 ≈ 25.6
b= (-1-30.29) ± √4×1×120
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