Question

In ΔABC, what is the value of cos B?

Answer 1

$b^{2} = a^{2} + c^{2} -2acCosB$
Making cos B the subject;
2acCosB=$a^{2} + c^{2} - b^{2}$
cosB=$\frac{ a^{2} + c^{2} - b^{2} }{2ac}$