还是√(x^2+a^2),暂且认为是后者吧
y=ln(x+√(x^2+a^2)),设u=√(x^2+a^2),则y=ln(x+u),则u'=1/2(x^2+a^2)^(-1/2)*2x=x(x^2+a^2)^(-1/2)
y'=(1+u')/(x+u)=[1+x(x^2+a^2)^(-1/2)]/[x+√(x^2+a^2)] =1/√(x^2+a^2)