In the sequence problem, you should always pay attention to the value of n term. If all your formulas are obtained on the premise of n≥2, you need to verify that n= 1.

In addition, there is an emergency measure. Whether it needs to be tested or not, you can't be wrong. Therefore, if you are preparing for the college entrance examination, you can take it under any circumstances to ensure that there will be no mistakes. However, if time permits, you should really understand.