/sci/ - Science & Math

I'm reading Discrete Mathematics and Its Applications (Kenneth Rosen) and half the explanations and proofs go right over my head. Am I a brainlet or is the book bad? Reviews on amazon seem bad too but this is what my university uses and others seem to recommend it.

At this point I'm just memorizing theorems/formulas without knowing why they work because I don't understand the explanations. Here's a prime example. I just googled the inclusion/exclusion principle for a much simpler explanation: just sum the lengths of the sets, then alternate between subtracting and adding the sets of the (2..n)-ary disjoint pairs between the sets.