Mathematics - Properties of Triangle Question with Solution | TestHub

Using the cosine rule, $\cos B=\frac{a^2+c^2-b^2}{2ac}$. Substituting the given values, $\cos B=\frac{5^2+8^2-7^2}{2(5)(8)}=\frac{25+64-49}{80}=\frac{89-49}{80}=\frac{40}{80}=\frac{1}{2}$.