By default, of course not. Unless the title tells us that A∪B=U, we should analyze it in detail. There won't be such a loophole in a large-scale exam unless the teacher usually does exercises for the students, so don't worry, you can definitely analyze whether there is a ∪ b = u-you are really a good student with a good brain. This is what you should do when you study. If you think wisely, you will become better. I really envy your teacher having a disciple like you. I wish you success in your study.