$0=1$, or I have done something wrong.

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$$\int \frac{1}{x}dx=\int1dx \frac{1}{x}-\int \left(\frac{d}{dx}\frac{1}{x}\int1dx\right)dx=1+\int\frac{1}{x}dx \Leftrightarrow 0=1$$

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yes! This is one of the integrals you should always remember! Recall that $$\frac{d}{dt}\left(\ln(x)\right)=\frac{1}{x}$$

hence $$\int \frac{1}{x} dx = \ln(|x|) + Constant$$

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When $f$ is continuous the "$=$" sign in $g(x)=\int f(x)dx$ is not literally "equals". It means that $g$ is an anti-derivative of $f.$ But $g+C$ is also an anti-derivative of $f$ for any constant $C.$