Logarithmic derivative

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${\displaystyle \int _{a}^{b}f'(t)\,dt=f(b)-f(a)}$
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In mathematics, specifically in calculus and complex analysis, the logarithmic derivative of a function f is defined by the formula $${\displaystyle {\frac {f'}{f}}}$$ where f′ is the derivative of f.^[1] Intuitively, this is the infinitesimal relative change in f; that is, the infinitesimal absolute change in f, namely f′ scaled by the current value of f.

When f is a function f(x) of a real variable x, and takes real, strictly positive values, this is equal to the derivative of ln f(x), or the natural logarithm of f. This follows directly from the chain rule:^[1] $${\displaystyle {\frac {d}{dx}}\ln f(x)={\frac {1}{f(x)}}{\frac {df(x)}{dx}}}$$