Avem formula [tex]tg(a+b)=\dfrac{\sin(a+b)}{\cos a\cos b}[/tex]
Obtinem:
[tex]\dfrac{\sin(x+3x+\dfrac{\pi}{4})}{\cos x\ \cos(3x+\dfrac{\pi}{4})}=0\Rightarrow \sin(4x+\dfrac{\pi}{4})=0[/tex] cu solutiile
[tex]4x_k+\dfrac{\pi}{4}=k\pi,\ k\in Z\Rightarrow x_k=\dfrac{k\pi}{4}-\dfrac{\pi}{16},\ k\in Z.[/tex]
