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Mathematics · 术语表

什么是 Natural logarithm?

又称: logarithm

Definition 23.5 High School Mathematics · 第 23 章 — Exponential and Logarithm

The exponential is continuous and strictly increasing from R\R onto (0, +∞)\intoo{0}{+\infty}; by the bijection theorem (Theorem 21.15), for every y>0y > 0 the equation ex=y\eu^x = y has a unique solution. This solution is the natural logarithm of yy, written ln⁡y\ln y. Thus

for x∈R, y>0:y=ex  ⟺  x=ln⁡y.\text{for } x \in \R,\ y > 0: \qquad y = \eu^x \iff x = \ln y .

In particular ln⁡1=0\ln 1 = 0, ln⁡e=1\ln \eu = 1, and eln⁡y=y\eu^{\ln y} = y, ln⁡(ex)=x\ln(\eu^x) = x.

The curves of  and  are mirror images of each other in the line y = x: the point (x, x) reflects to ( x, x).
The curves of exp⁡\exp and ln⁡\ln are mirror images of each other in the line y=xy = x: the point (x,ex)(x, \eu^x) reflects to (ex,x)(\eu^x, x).
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