3}\] \[\Delta =\FRAC{1}{2}ab\sin C\Rightarrow ab=20\sqrt{3}\frac{2}{\sqrt{3}}=40\] ?.. (i) Also \[a+b+c=20\]or \[a+b=(20-c)\] Now, \[\cos C=\frac{{{a}^{2}}+{{b}^{2}}-{{c}^{2}}}{2AB}=\frac{1}{2}\] Þ \[{{a}^{2}}+{{b}^{2}}-{{c}^{2}}=ab\] Þ \[{{(a+b)}^{2}}-{{c}^{2}}=ab+2ab=3ab\] Þ \[{{(20-c)}^{2}}-{{c}^{2}}=3(40)\] Þ \[-40c+400=120\Rightarrow c=7\].