Let ∆=|\table \sin \theta \cos \phi , \sin \theta \sin \phi , \cos \theta ; \cos \theta \cos \phi , \cos \theta \sin \phi , -\sin \theta ; -\sin \theta \sin \phi , \sin \theta \cos \phi , 0| Then

( { {d} ∆}/{ {d} \theta})_{\theta=\frac{\pi}{2}}=0