PLEASE HELP
verify the identity tan(x1+x2+x3)=tan(x1)+tan(x2)+tan(x3)-tan(x1)tan(x2)tan(x3)/1-tan(x1)tan(x2)-tan(x2)tan(x3)-tan(x1)tan(x3), what should you do first?

Please dont answer just to get points please only answer if you know the answer.