x → 1 lim ( 1 − x ) n − 1 ( 1 − x ) ⋅ ( 1 − 3 x ) ⋯ ( 1 − n x ) = ?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Nice observation! There are other ways to get the solution without substituting. Try to get this: n → ∞ lim n ! 1 . You could also use "The Squeeze Theorem".
I substituted n=1.
Log in to reply
You can't solve the problem only with substituting n=1, but you could proove by induction that it works for all n.
Log in to reply
Here is my proof by induction: Let P n be the proposition that x → 1 lim ( 1 − x ) n − 1 ( 1 − x ) ⋅ ( 1 − 3 x ) ⋯ ( 1 − n x ) = 0 for positive integer n.
For base case n=1:
LHS= lim x → 1 ( 1 − x ) 0 ( 1 − x ) = 0
Assume P k is true for some positive integer k.
Then, when n = k + 1 :
LHS= x → 1 lim ( 1 − x ) k ( 1 − x ) ⋅ ( 1 − 3 x ) ⋯ ( 1 − k x ) ( 1 − k + 1 x ) = x → 1 lim ( 1 − x ) k − 1 ( 1 − x ) ⋅ ( 1 − 3 x ) ⋯ ( 1 − k x ) ⋅ x k 1 − k + 1 x
= x → 1 lim ( 1 − x ) k − 1 ( 1 − x ) ⋅ ( 1 − 3 x ) ⋯ ( 1 − k x ) ⋅ x → 1 lim x k 1 − k + 1 x
= 0
Hence, by Mathematical Induction, since P 1 , P k , P k + 1 are true, the proof is complete.
Log in to reply
@A Former Brilliant Member – That's it :)
Log in to reply
@Aleksa Radovanović – I'm not sure that induction can be applied here. Since the numerator starts with ( 1 − x ) the case n = 1 doesn't really make sense. For n = 2 the limit is
lim x → 1 1 − x 1 − x = lim x → 1 ( 1 − x ) ( 1 + x ) 1 − x = 2 1 .
In general, lim x → 1 1 − x 1 − x n 1 = n 1 for any non-zero integer.
So the given limit can be written as k = 2 ∏ n k 1 = n ! 1 , which would only go to 0 as n → ∞ .
Log in to reply
@Brian Charlesworth – Yes. I agree with your logic.
@Brian Charlesworth – Yes, did same. Also, putting n=1 does not make sense here.
Problem Loading...
Note Loading...
Set Loading...
The limit above simplifies to x → 1 lim 1 − x = 0 because 1 − n x = 1 − x for all n when substituted for x = 1