In a Δ ABC, a = 3, b = 5 and c = 7, find the values of cos A, cos B,

1.	In a Δ ABC, a = 3, b = 5 and c = 7, find the values of cos A, cos B, cos C.
Answer» 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 = \(\frac{a^2+b^2-c^2}{2ab}\) \(=\frac{3^2+5^2-7^2}{2.3.5}\) \(=\frac{9+25-49}{30}\) \(=-\frac{15}{30}\) \(=-\frac{1}{2}\)

In a Δ ABC, a = 3, b = 5 and c = 7, find the values of cos A, cos B, cos C.

Answer»

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 = \(\frac{a^2+b^2-c^2}{2ab}\)

\(=\frac{3^2+5^2-7^2}{2.3.5}\)

\(=\frac{9+25-49}{30}\)

\(=-\frac{15}{30}\)

\(=-\frac{1}{2}\)