y=Arctan[x-sqrt(x^2+1)]
tany=x-sqrt(x^2+1)
y'*(secy)^2=1-(2x)/[2sqrt(x^2+1)]
y'*[(tany)^2+1]=1-x/sqrt(x^2+1)
y'*([x-sqrt(x^2+1)]^2+1)=(sqrt(x^2+1)-x)/sqrt(x^2+1)
y'*(2x^2-2xsqrt(x^2+1)+2)=(sqrt(x^2+1)-x)/sqrt(x^2+1)
y'=(sqrt(x^2+1)-x)/[sqrt(x^2+1)*(2x^2-2xsqrt(x^2+1)+2)]