Oh, okay, I'll answer that. There is an answer under the topic, so I won't talk about the solution. As for dislocation subtraction, because an/bn is the quotient of geometric progression and arithmetic progression, after writing Sn, multiply Sn by the common ratio Q, then align the second term of the following formula with the first term and write it in turn. There will be one more item in front of the above formula, and one more Q after the following formula.

An/bn, subtract the following formula from the above formula, and the difference between the middle terms becomes a geometric series, and then calculate it with the equal ratio formula, okay? It looks troublesome. Let's get started. Easy to master. Let's go