#y = arctan sqrt [ (1-x)/(1+x)]#
#tan y = sqrt [ (1-x)/(1+x)]#
#tan^2 y = (1-x)/(1+x)#
#tan^2 y = 2/(1+x) - 1#
differentiating each side
#2 tan y sec^2 y\ y' = -2/(1+x)^2#
# tan y (tan^2 y + 1)\ y' = -1/(1+x)^2#
# sqrt [ (1-x)/(1+x)] ( (1-x)/(1+x) + 1)\ y' = -1/(1+x)^2#
# sqrt [ (1-x)/(1+x)] ( (2)/(1+x))\ y' = -1/(1+x)^2#
# \ y' = - 1/(1+x)^2 * sqrt [ (1+x)/(1-x)] * (1+x)/2#
# \ y' = - 1/(2 sqrt(1+x) * sqrt [(1-x)] #
# \ y' = - 1/(2 sqrt (1-x^2) #