The direction ratios of two lines are given by$〈p,q,r〉\text{\hspace{0.17em}\hspace{0.17em}and\hspace{0.17em}\hspace{0.17em}}〈\frac{1}{qr},\frac{1}{rp},\frac{1}{pq}〉$. Ifis the angle between them such that $0\le \alpha <\pi$, then the value of  $\cos \alpha +{\cos }^2\alpha +{\cos }^3\alpha +\mathrm{....}+{\cos }^p\alpha \cdot {\cos }^q\alpha \cdot {\cos }^r\alpha =$

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