3．The angle \(\theta , 0 < \theta < \frac { \pi } { 2 }\) ，satisfies $$\tan \theta \tan 2 \theta = \sum _ { r = 0 } ^ { \infty } 2 \cos ^ { r } 2 \theta$$ （a）Show that \(\tan \theta = 3 ^ { p }\) ，where \(p\) is a rational number to be found．
（b）Hence show that \(\frac { \pi } { 6 } < \theta < \frac { \pi } { 4 }\)