Question

In a ∆ABC, if c² + a² – b² = ac, then the measure of angle B is ____________.

Answer 1

In a ∆ABC,
if c² + a² − b² = ac

$\begin{array}{rcl}\mathrm{Since}\cos B& =& \frac{c^2+a^2-b^2}{2ac}\\ & =& \frac{ac}{2ac}\end{array}$

i.e cosB = $\frac{1}{2}$

i.e B =$\frac{\mathrm{\pi }}{3}$