y = x/√(1-x^2)
y' = [√(1-x^2) + 0.5*2x^2/√(1-x^2)]/(1-x^2)
= [√(1-x^2) + x^2/√(1-x^2)]/(1-x^2)
= [(1-x^2) + x^2]/(1-x^2)^(2/3)
= 1 / (1-x^2)^(2/3)