Answer :
From double-angle formula we know that:
[tex]\sin(2\theta)=2\sin\theta\cos\theta[/tex]
so (for θ = c/2):
[tex]\sin c=\sin(2\cdot\frac{c}{2})=2\sin(\frac{c}{2})\cos(\frac{c}{2})[/tex]
[tex]\sin(2\theta)=2\sin\theta\cos\theta[/tex]
so (for θ = c/2):
[tex]\sin c=\sin(2\cdot\frac{c}{2})=2\sin(\frac{c}{2})\cos(\frac{c}{2})[/tex]