y = -x/√(a^2-x^2)y'={1*（a^2-x^2)^(1/2)-x*(1/2)[(a^2-x^2)^(-1/2)]*(-2x)*1}/[(x^2-a^2)^(1/2)^2y'=[(a^2-x^2)^(1/2)+x^2*(a^2-x^2)^(-1/2)]/[(a^2-x^2)=[(a^2-x^2)+x^2]/(a^2-x^2)^(3/2)故,y'=a^2/(a^2-x^2)^(3/2)
