y'=[x/√(x^2+1)]/[1+√(x^2+1)]=[x/√(x^2+1)] [√(x^2+1)-1]/x^2=(x^2+1)/x-√(x^2+1)/x=x+1/x-√(x^2+1)/xy''=1-1/x^2-[1/√(x^2+1)-√(x^2+1)/x^2)以上回答你满意么？
