In high school mathematics, when writing series, under what circumstances should we replace the first item for testing?
In the same term of sn, n is greater than 1. Through this, we can find the same term of s(n- 1). In this process, n is replaced by N- 1, so n is greater than 2. The an obtained in this way is a general formula with n greater than 2, so we need to substitute the first item into the same item to see if it is appropriate, and write one if it is appropriate.