The best device combinations for studying(e-ink, tablet and laptop) and guide for buying other stuffs

This is the basic device combinations that I found it performs the best in less distraction, more focus, good portability, and productivity for 9-5 studying

• The "6-inches" screen E-ink that compatible with KOreader (mostly Android OS)
  • Use for reading miscellaneous books in free time
  • The 6 or 6.8 -inches screen is the best. Any screen larger than 6.8 inches is a drawback, because to be able to read in free time (eg. traveling, eating, etc.), the ability to be able to hold in one hand securely and comfortably is the most important.
  • KOreader app also provides the sophisticated reflow function and cropping tools that make reading any type of books become possible, even in just a small hand size e-reader.
    • Thus, even though the more screen estate seem providing more benefit, any e-ink screen larger than 6 inches has no significant benefit(but drawback) in reading.
  • Don't try or expect using large screen e-ink for reading complex material, it is completely useless, just a gimmick. Because even though it is large, it is still not large enough for many complex textbooks, and still need to be zoom or reflow which also inhibit note taking and resulting in no different from the 6-inches screen, plus poorer portability.
• Tablet which has High screen resolution and good color accuracy with pen
  • Use for more complex documents reading which require color and note taking(e-ink never does these jobs well)
    • * However, in case that you don't use pen much(no lecture noting or drawing work), a tablet would be redundant to a small lightweight laptop.
  • Many mid-range tablets nowadays surpass those requirements
  • Don't buy the low screen resolution(less than 200 dpi) tablet because it will not differ from common laptop screen, harder to read, and has no benefit increased from redundancy.
• Laptop with light weight, 14 inches screen, good battery life(low performance just on the go) are enough.
  • Use for working, searching deeply for information, doing anything much more complex than the tablet can provide(even for comparing shopping items).
  • The main purpose for laptop is portability, so it should have light weight(should not more than 1.5kg)
  • I found that 14 inches screen is more than enough for all productive works.
  • Moderate or low performance is enough, any high performance laptop is useless and waste of money because laptop has poor heat dissipation that limits the performance.
  • No need to use expensive laptop that provide little longer battery life. Just those moderate or low performance usually provide decent battery life, because of less energy consumption.
    • If your work specifically requires higher performance, consider another desktop PC.
    • Also, buying a good second handed laptop is also a good choice. Because you can get even a high-end laptop in a very cheap price in a very good condition.

My stuff buying guide

• Buy only when it is needed, not just for trying or sparing.
• Buy only the good quality stuff, for long lifespan and less waste.
  •  The 2nd hand good quality stuff is also excellent choice, at least it saves your money and the earth. 

Another gadgets(but not for productivity purpose)

• Smartphone - the main purpose is the decent portable camera, not sure about AI or any other function but if the phone has well-built cameras, the other functions usually be good either. 
• Ear phone - should have active noise cancelling, water resistance