FRAC{\sin 3B}{\sin B}=\frac{3\sin B-4{{\sin }^{3}}B}{\sin B}=3-4{{\sin }^{2}}B\] \[=3-4+4{{\cos }^{2}}B=-1+\frac{4{{({{a}^{2}}+{{C}^{2}}-{{b}^{2}})}^{2}}}{4{{(ac)}^{2}}}\] \[=-1+\frac{{{\left( \frac{{{a}^{2}}+{{c}^{2}}}{2} \RIGHT)}^{2}}}{{{(ac)}^{2}}}=-1+\frac{{{({{a}^{2}}+{{c}^{2}})}^{2}}}{4{{(ac)}^{2}}}\] \[=\frac{{{({{a}^{2}}+{{c}^{2}})}^{2}}-4{{a}^{2}}{{c}^{2}}}{4{{(ac)}^{2}}}={{\left( \frac{{{c}^{2}}-{{a}^{2}}}{2ac} \right)}^{2}}\].