Ответ:

[tex] {c}^{2} = {a}^{2} + {b}^{2} - 2ab \cos( \gamma ) [/tex]

[tex] {c}^{2} = {5}^{2} + {4}^{2} - 2 \times 5 \times 4 \times \cos( {60}^{ \circ} ) \\ {c}^{2} = 25 + 16 - 2 \times 20 \times \frac{1}{2} \\ {c}^{2} = 41 - 20 \\ {c}^{2} = 21 \\ c = \sqrt{21} [/tex]