$I_n =\int \limits_0^{z/4} \tan ^n x \;dx$ find $I_{n-1}+I_{n+1}$

Question

\[\begin {array} {1 1} (a)\;\frac{2}{n} \\ (b)\;\frac{1}{n} \\ (c)\;\frac{\pi}{n} \\ (d)\;\frac{\pi}{4n} \end {array}\]