tan(y)=tan(π-(a+B))=-tan(a+B), so tan(a)+tan(B)+tan(y)=tan(a)+tan(B)-tan(a+B).
tan(y)=-tan(a+B)=-(tan(a)+tan(B))/(1-tan(a)tan(B)),
tan(y)-tan(y)tan(a)tan(B)=-(tan(a)+tan(B)),
tan(y)+(tan(a)+tan(B))=tan(y)tan(a)tan(B), which is the same as:
tan(a)+tan(B)+tan(y)=tan(a)tan(B)tan(y) QED