half angle formula for tangent of an angle
\[tan(\frac{x}{2})\]\[=\sqrt{\frac{1-cosx}{1+cosx}}\]
Proof-
\[ 1-cosx=2sin^2{\frac{x}{2}}\]
\[ 1+cosx=2cos^2{\frac{x}{2}}\]
RHS=\[\sqrt{\frac{1-cosx}{1+cosx}}=\sqrt{\frac{2sin^2\frac{x}{2}}{2cos^2\frac{x}{2}}}\]=\[ \sqrt{tan^2\frac{x}{2}}=+ or -tan\frac{x}{2}\]
