On the one hand, how can the source of this topic and its authoritative accuracy be unknown? On the other hand, watching the mobile phone for a long time is not good for your eyesight, so it is inevitable to draw a draft and write some conceptual formulas in the process of doing the problem. It is difficult to do this with a mobile phone or a tablet. If you use a paper book, you can write the answers and procedures in the book, which is also conducive to review and review in the future.