求导后是1/√(1+x^2)＝(1+x^2)^(-1/2)＝1＋(-1/2)*x^2＋...＋[(-1)^n*1/2*3/2*...*(2n-1)/2]/n!*x^(2n)＋...＝1 ＋ ∑[(-1)^n*(2n-1)!/(2n)!]*x^(2n),n从1到∞
再积分,ln[x+√(x^2+1)]＝x ＋ ∑[(-1)^n*(2n-1)!/((2n)!×(2n+1))]*x^(2n+1),n从1到∞.收敛域是[-1,1]