case 1
\[tan^{-1}(x)+tan^{-1}(y)=tan^{-1}(\frac{x+y}{1-xy})\]
boundary condition - xy<1
example for x= 0.5 and y =1/3 =0.333333 we get 45 degrees
case 2 when x>0 y>0 and xy>1
\[tan^{-1}(x)+tan^{-1}(y)=180^{0}+tan^{-1}(\frac{x+y}{1-xy\frac{}{}})\]
example x=2 and y =3 we get 135 degrees
case 3 x<0 y < 0 and xy>1
\[tan^{-1}(x)+tan^{-1}(y)=-180^{0}+tan^{-1}(\frac{x+y}{1-xy\frac{}{}})\]
if x=-2 , y=-3 and xy > 1 we get -135 degrees
