テイラー展開
log  ( 1 + x ) = ∑ n = 1 ∞ ( − 1 ) n + 1 n x n for | x | < 1 {\displaystyle \log(1+x)=\sum _{n=1}^{\infty }{\frac {(-1)^{n+1}}{n}}x^{n}\quad {\mbox{ for }}|x|<1}