## Trigonometry (11th Edition) Clone

$A$, $B$ and $C$ being 3 angles of a triangle means that the sum of them equals $180^\circ$. Therefore, $\sin(A+B+C)$ equals $\sin180^\circ$, which equals $0$.
If $A$, $B$ and $C$ are the 3 angles of a triangle, that means $$A+B+C=180^\circ$$ Therefore, $$\sin(A+B+C)=\sin180^\circ=0$$