I need to solve
I can easily spot that I need to substitute
$$u=\tan\theta$$ $$du=\sec^2\theta d\theta$$
Which lead me to
However, if I look at the correction steps, I should be trying to solve instead
Now I tought maybe I've chosen a wrong substitution variable, but they actually use the same $u=\tan\theta$.
What is wrong with my substitution ?