For example:1[n (n-1)] = [1(n-1)]-(1/n)

Dislocation subtraction is a method to prove the first n terms and Sn of geometric series. Generally, write Sn, multiply both sides by the common ratio q, and subtract the last two expressions to get Sn.

PS: Well ... some things in mathematics can only be solved by mathematical symbols, so it's not clear. Dislocation subtraction is more difficult, I suggest you have a look.

/question/4895240 1.html I gave you the answer and explanation of the third question, hoping to help you (that is, the first question with the best answer).