The Hardest High School Math by Trigonometric Substitution in Vietnam - Integral with long series

Here is the technique to solve the integral

Integral ( 1 + tan^2 x)(1 + tan^3 x)...(1 + tan^2020 x) + ... + (1+tanx)(1 + tan^2 x)... (1 + tan^2019 x) / (1 + tanx)(1 + tan^2 x)(1 + tan^3 x) ... (1+tan^2020 x) dx

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