3.30 Given vectors \begin{array}{l} \mathbf{A}=\hat{\mathbf{r}}(\cos \phi+3 z)-\hat{\boldsymbol{\phi}}(2 r+4 \sin \phi)+\mathbf{z}(\boldsymbol{r}-2 z) \\ \mathbf{B}=-\mathbf{r} \sin \phi+\hat{\mathbf{t}} \cos \phi \end{array} find \text { (a) } \theta_{A B} \text { at }\left(2_{-} \pi

/ 2,0\right) \text { (b) A unit vector perpendieular to both } A \text { and } B \text { at }(2, \pi / 3,1)