In a ΔABC, if a = 3 cm, b = 5 cm and c = 7 cm,

Given: a = 3 cm, b = 5 cm and c = 7 cm

Need to find: cos A, cos B, cos C

$\cos A=\frac{b^{2}+c^{2}-a^{2}}{2 b c}=\frac{5^{2}+7^{2}-3^{2}}{2.5 .7}=\frac{65}{70}=\frac{13}{14}$

$\cos B=\frac{c^{2}+a^{2}-b^{2}}{2 c a}=\frac{7^{2}+3^{2}-5^{2}}{2.7 .3}=\frac{33}{42}$

$\cos C=\frac{a^{2}+b^{2}-c^{2}}{2 a b}=\frac{3^{2}+5^{2}-7^{2}}{2.3 .5}=\frac{-15}{30}=-\frac{1}{2}$