11R.H.S = 4\ cos\ (\frac{\pi }{4} - \frac{A}{4})\ cos\ (\frac{\pi }{4} - \frac{B}{4})\ cos\ (\frac{\pi }{4} - \frac{C}{4})
= \frac{4}{\sqrt{2}\sqrt{2}\sqrt{2}} \left[cos\frac{A}{4}\ + Sin\ \frac{A}{4} \right]\left[cos\frac{B}{4}\ + Sin\ \frac{B}{4} \right]\left[cos\frac{C}{4}\ + Sin\ \frac{C}{4} \right]
= \sqrt{2} \left[Sin\ (\frac{A}{4}+\frac{B}{4}+\frac{C}{4}) + Cos\(\frac{A}{4}+\frac{B}{4}+\frac{C}{4}) \right]
= 2\ Sin\ (\frac{A}{2} + \frac{B}{2} + \frac{C}{2})
What to do next ? [2]
