Răspuns:
Explicație pas cu pas:
[tex]sinx+siny=2*sin\frac{x+y}{2}*cos\frac{x-y}{2}\\sin(x+y)=sin(2*\frac{x+y}{2})=2*sin\frac{x+y}{2}*cos\frac{x+y}{2}.\\deci~sinx+siny+sin(x+y)=2*sin\frac{x+y}{2}*cos\frac{x-y}{2}+2*sin\frac{x+y}{2}*cos\frac{x+y}{2}=2*sin\frac{x+y}{2}*(cos\frac{x-y}{2}+cos\frac{x+y}{2}).[/tex]