"Book of Proof" by Richard Hammack (free online). This is more gentle than Velleman but excellent for drilling.
Try a proof by contradiction.
The basic language of modern math, including operations like unions, intersections, and complements. Proof Techniques: 18.090 introduction to mathematical reasoning mit
Students must have completed 18.01 (Single Variable Calculus) . "Book of Proof" by Richard Hammack (free online)
The course moves away from "finding an answer" toward "proving why it's true." To prepare, focus on these core areas: Logic Fundamentals including operations like unions