[tex] ac {}^{2} = ab {}^{2} + bc {}^{2} - 2 \times ab \times bc \times \cos( \beta ) \\ 2 \times ab \times bc \times \cos( \beta ) = ab {}^{2} + bc {}^{2} - ac {}^{2} \\ \cos( \beta ) = \frac{ab {}^{2} + bc {}^{2} - ac {}^{2}}{2 \times ab \times bc} \\ \cos( \beta ) = \frac{14 {}^{2} +2 {}^{2} - 13 {}^{2} }{2 \times 14 \times 2} = \frac{196 + 4 - 169}{56} = \frac{31}{56} [/tex]