A , B , C are integers satisfying A B C = 2 2 2 . One possible solution is ( A , B , C ) = ( 2 , 2 , 2 ) .
How many other solutions ( A , B , C ) are there?
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A comment asks about about the order that exponent towers are calculated.
Exponent towers are calculated from the top down. The reason for this is consistency. Consider:
2 3 + 1
There is an "implicit parentheses" where the addition is grouped together. The 3 + 1 happens first, and then 2 4 occurs.
In order to be consistent, this grouping has to happen with the exponents as well. With 2 3 4 , the 3 4 needs to happen first, since it is grouped in the same way as 3 + 1 in the previous expression.
Yay! I love the last one!
It might just be because it's 1:20am here, but I don't get the last one. Can someone please explain?
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A B C = ( 1 6 n m ) n − m = 1 6 n m × n − m = 1 6 n m − m = 1 6 n 0 = 1 6 1 = 1 6
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Yes I get that. It's neat, but it's not the solution offered by David. David's solution has B = n and C = -m. Joseph's solution has B = n to the power of m and C = n to the power of -m. In the latter case C is not an integer.
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@Paul Cockburn – No, Joseph and mine are the same: A = 1 6 n m , B = n , and C = − m .
I'm under the assumption that powers are calculated from the bottom up:
16^2^0 = 256^0 = 1
But you're implying that they're calculated top-down:
16^2^0 = 16^1 = 16
Clearly you're right, but why? It seems arbitrary to do it one way or another.
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Here's a good article explaining exponent towers: https://brilliant.org/wiki/what-is-a-to-the-b-to-the-c/
If you do it bottom up (a^b)^c=a^(b*c). As such, exponent towers would not really be useful, as a simpler notation would exist, with just one exponentiation step, and (possibly several) multiplications.
I hope this can help as a reminder why it is the way it is...
Surely the 3rd option doesn't work as written. e.g. for n=2, m=1 A^(B^(C))=16^(2^(-1))=16^(0.5)=4
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If n = 2 and m = 1 for the third option, A = 1 6 n m = 1 6 2 1 = 1 6 2 = 2 5 6 . Then A B C = 2 5 6 2 − 1 = 2 5 6 2 1 = 1 6 .
In the 3rd case, if m is a negative integer, A need not be an integer.
For example, if n=3 and m= -1, A = 16^3^(-1) = 16^(1/3) = Cube root of 16, which is not an integer.
Apart from special cases, the 3rd case scenario should work if n and m are positive integers. Or if n is negative and m is positive and even.
Another scenario that shows there are infinitely many solutions:
A=16, B=0, C=n for any integer n
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No, because then A B C = 1 6 0 n = 1 6 0 = 1 , not 1 6 .
For any integer N , 1 6 1 N = 2 2 2 , which yields the infinitely many solutions given by ( A , B , C ) = ( 1 6 , 1 , N )
Nice. There are other general solutions as well. I hope that other people can "find them all"!
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Sure, and if anyone wants to enumerate through all the solutions, [joke follows] my advice is to condition on the value of B . I've already done B = 1 , so there's only infinitely more values of B remaining to check! Easy peasy!
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But we still can find infinitely many solutions where A = 1 6 .
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@Pi Han Goh – Shhhhhhh! You might convince people it's not "easy peasy"!
But seriously, I actually think conditioning on A is better. Then just work up the tower
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@Brian Moehring – Sorry for being a buzzkill :(
Currently, there are 5 attempts, only one (you) got this right. I'm pretty proud of this question!
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@Pi Han Goh – Yes, great question. I wish I could "Like" it, but alas, that is no longer an option. :(
@Pi Han Goh – I'm only finding +/- variations of 2 4 1 and 4 2 1 , in addition to those already mentioned.
Edit: Oh wait, there are powers like 2 5 6 2 − 1 , 4 0 9 6 3 − 1 , and on up. So yeah, infinitely many .... :)
2 2 2 = 1 6 = 1 6 B 0 , where B can be any integer except 0, so long as A = 1 6 and C = 0 . Therefore, there are infinitely many other solutions.
Of course I shoud have seen that!
For ( A , B , C ) = ( 2 2 0 x , 5 x , − 1 )
A B C = ( 2 2 0 x ) ( 5 x ) − 1 = ( 2 2 0 x ) 5 x 1 = 2 2 0 x × 5 x 1 = 2 4 = 2 2 2
Where x can be any non-zero integer. Hence there are infinitely many solutions for ( A , B , C ) .
I admire your solution, because I wouldn't have thought of such solution. Didn't even occur to me to think out of the box.
2^2^2 = 16 = 16^1^n = 16^k^0 where n is any integer and k is any integer NOT equal to 1. So we have infinitely many solutions
For any non-zero integer n ,
2 2 2 = 1 6 = 1 6 n 0
Therefore there are infinitely many solutions.
For Non-Zero integer Z:
A= 16^Z
B= Z
C= -1
so it is Zth root of 16^Z which is 16
That’s not how it works- A^B^C = A^(B^C) ,not (A^B)^C
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Many thanks for responding. But what's wrong with my solution:
as you say A^B^C = A^(B^C)
as I have proposed:
A= 16^Z
B= Z
C= -1
So (B^C) = Z^(-1) = 1 / Z
Then A^(B^C) = (16^Z)^(1/Z)
That is Zth root of (16^Z), which is 16
Am I wrong?
1 6 B 0 = 2 2 2 for all nonzero integers B .
Ultimately we have to get 16. Let's say C=1, so we can forget about it. Now, we know that we can get to any number by putting some number to some power, so we already know, that the answer is infinitely many. For eg. we can take our calculator and input sth like: log840(16) We get: 840^0,4117664097^1=16 So: A=840 B=0,4117664097 C=1
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Here are a few variations that would yield infinitely many solutions:
A = 1 6 , B = 1 , and C = n for any integer n
A = 1 6 , B = n , and C = 0 for any non-zero integer n
A = 1 6 n m , B = n , and C = − m for any non-zero integers m and n