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1. The formula is written incorrectly.
Fix and edit it.
2. As I just said it to you in my previous post,
( https://www.algebra.com/algebra/homework/Sequences-and-series/Sequences-and-series.faq.question.1044033.html )
learn about the method of mathematical induction from the lessons
- Mathematical induction and arithmetic progressions
- Mathematical induction and geometric progressions
- Mathematical induction for sequences other than arithmetic or geometric
in this site.
Then try to solve this problem on your own.
It is the simplest problem on this method.