How to find the third side in a triangle if one side is 3 cm, the second is 4 cm and the angle between them is 120 degrees.

univerkov | January 13, 2021 | education

If in a triangle side a = 3 cm; side b = 4 cm, the angle between them C = 120 °, the third side c can be calculated by the formula:
c² = a² + b² – 2 a b cos C;
cos 120 ° = – 0.5;
c² = 3² + 4² – 2 * 3 * 4 * (- 0.5) = 9 + 16 + 12 = 37;
c = √ 37 (cm) ≈ 6.08 cm.

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