y=(a^2)/√(a^2-x^2)
=a^2*(a^2-x^2)^(-1/2)
y'=a^2*(-1/2)*(a^2-x^2)^(-1/2-1)*(a^2-x^2)'
=-1/2*a^2*(a^2-x^2)^(-3/2)*(-2x)
=a^2x/√(a^2-x^2)^3