9. В классе учится 25 человек, из них 18 человек посещают математический кружок, а 12 — кружок по астрономии. Выберите верные утверждения и запишите в ответе их номера.

Let M be the set of students attending the math club and A be the set of students attending the astronomy club. |Total| = 25, |M| = 18, |A| = 12.
Using the principle of inclusion-exclusion: |M U A| = |M| + |A| - |M ∩ A|
Since |M U A| <= 25, we have 18 + 12 - |M ∩ A| <= 25, which means 30 - |M ∩ A| <= 25, so |M ∩ A| >= 5.
1) False, because |M ∩ A| can be less than 25.
2) True, since |M ∩ A| >= 5, there are at least 5 students attending both.
3) False, if |M ∩ A| = 5, then 18 - 5 = 13 attend only math.
4) True, since |M ∩ A| >= 5, which is less than 13.