B\]and \[\cot C\]are in A. P. Þ \[\cot A+\cot C=2\cot B\] Þ \[\frac{\cos A}{\SIN A}+\frac{\cos C}{\sin C}=\frac{2\cos B}{\sin B}\] Þ \[\frac{{{b}^{2}}+{{c}^{2}}-{{a}^{2}}}{2bc(ka)}+\frac{{{a}^{2}}+{{b}^{2}}-{{c}^{2}}}{2ab(kc)}=2\frac{{{a}^{2}}+{{c}^{2}}-{{b}^{2}}}{2ac(kb)}\] Þ \[{{a}^{2}}+{{c}^{2}}=2{{b}^{2}}\]. HENCE\[{{a}^{2}},{{b}^{2}},{{c}^{2}}\]are in A. P. NOTE : Students should remember this question as a FACT.