y = arctan[(x+1)/(x-1)]y' = 1 / {1+[(x+1)/(x-1)]²} * [(x+1)' (x-1) - (x+1) (x-1)'] / (x-1)² = (x-1)² / [(x-1)²+(x+1)²] * (-2)/(x-1)² = -1/(x²+1)
