cos A = \(\frac{b^2+c^2-a^2}{2bc}\)
\(=\frac{5^2+7^2-3^2}{2.5.7}\)
\(=\frac{25+49-9}{70}\)
\(=\frac{65}{70}\)
\(=\frac{13}{14}\)
cos B = \(\frac{a^2+c^2-b^2}{2bc}\)
\(=\frac{3^2+7^2-5^2}{2.3.7}\)
\(=\frac{9+49-25}{42}\)
\(=\frac{33}{42}\)
\(=\frac{11}{14}\)
cos C = \(\frac{a^2+b^2-c^2}{2ab}\)
\(=\frac{3^2+5^2-7^2}{2.3.5}\)
\(=\frac{9+25-49}{30}\)
\(=-\frac{15}{30}\)
\(=-\frac{1}{2}\)
