[tex]\text{Folosesti formula:} \\
\boxed{\cos(a+b)=\cos a\cdot \cos b-\sin a\cdot \sin b}\\
\text{In cazul nostru:}\\
\sin(\pi -x)\cdot \sin x-\cos (\pi -x)\cdot \cos x=\\
=-(\cos (\pi -x)\cdot \cos x-\sin(\pi -x)\cdot \sin x)=-\cos ( (\pi -x)+x)=\\
=-\cos\pi =-1[/tex]