C are in A. P. then angle\[B={{60}^{o}},\] \[\cos B=\frac{{{a}^{2}}+{{c}^{2}}-{{b}^{2}}}{2ac}\], \[\left\{ \begin{align} & \text{since }A+B+C={{180}^{o}}\,\,\text{and} \\ & \text{ }A+C=2B\RIGHTARROW B={{60}^{o}} \\ \END{align} \right\}\] Þ \[\frac{1}{2}=\frac{{{a}^{2}}+{{c}^{2}}-{{b}^{2}}}{2ac}\Rightarrow {{a}^{2}}+{{c}^{2}}-{{b}^{2}}=ac\] Þ \[{{b}^{2}}={{a}^{2}}+{{c}^{2}}-ac\].