If $3\tan \left(\theta -15°\right)=\tan \left(\theta +15°\right),0<\theta <\pi$ , then $\theta =$

$3\frac{\sin \left(\theta -15°\right)}{\cos \left(\theta -15°\right)}=\frac{\sin \left(\theta +15°\right)}{\cos \left(\theta +15°\right)}$

$\frac{3}{2}\times 2\cos \left(\theta +15°\right)\sin \left(\theta -15°\right)=\frac{2}{2}\sin \left(\theta +15°\right)\cos \left(\theta -15°\right)$

$3\left(\sin 2\theta -\sin 30°\right)=\sin 2\theta +\sin 30°$

$2\sin 2\theta =4\sin 30°$

$\sin 2\theta =1$

$2\theta =\frac{\pi }{2}\Rightarrow \theta =\frac{\pi }{4}$