Chapter 8 - Application of Trigonometry - 8.2 The Law of Cosines - 8.2 Exercises - Page 766: 11

$120^0$

From the law of cosines $7^{2}=3^{2}+5^{2}-2\times 3\times 5\times \cos \theta \Rightarrow 49=34-30\times \cos \theta \Rightarrow -30\cos \theta =15\Rightarrow \cos \theta =-\dfrac {1}{2}\Rightarrow \theta =\cos ^{-1}\left( -\dfrac {1}{2}\right) =120^0$