# $\large\frac{d}{dx}$$\bigg[a \tan ^{-1} + b \log \bigg(\large\frac{x-1}{x+1}\bigg)\bigg]=\frac{1}{x^4-1}$$=>a-2b=$

$\begin {array} {1 1} (1)\;1 & \quad (2)\;-1 \\ (3)\;0 & \quad (4)\;2 \end {array}$

(2) -1 is the correct answer.