I found this interesting equality, but I could not find a way to prove it. Any (beautiful) idea? lim_(n-> oo)((root[n](a)+root[n](b))/(2))^n overset(?)(=)sqrt(ab)

Layton Park

Layton Park

Answered question

2022-11-03

lim n ( a n + b n 2 ) n = ? a b
I found this interesting equality, but I could not find a way to prove it. Any (beautiful) idea?
lim n ( a n + b n 2 ) n = a b

Answer & Explanation

barene55d

barene55d

Beginner2022-11-04Added 23 answers

Let
y = ( a n + b n 2 ) n
log y = n log a n + b n 2
Let n = 1 p as n , p 0
log y = lim p 0 log a p + b p 2 p
Applying L.H rule, we get
log y = lim p 0 a p log a + b p log b a p + b p
log y = 1 2 log a b
y = a b
atgnybo4fq

atgnybo4fq

Beginner2022-11-05Added 5 answers

Even though there are other solutions, I just wanted to give one that is pretty simple. If we could show that this sequence is decreasing (I suspect the devil is in the details on this one) we could just employ the Arithmetic-Geometric mean inequality. We'd find:
lim n ( a 1 / n + b 1 / n 2 ) n lim n a b = a b
Since this is a decreasing sequence bounded below, the limit must the infimum.

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