I have seen this question posted somewhere on this site before, but they included a lemma in which $$_{2−1}=F^2_+F^2_{−1}$$ This wasn't demonstrated to me through my textbook, and I was asked to prove the same question. So I am curious if there is another way to prove this question without such a lemma. I tried relating the $F_{2n}$ term without this lemma but failed, so I want to know whether or not this is possible without that lemma.
I am very new to number theory, so in addition to this question, can I get some advice on how to improve in this field of mathematics and how should I spot which lemma or theorems are needed for a question?