Which is the base? Which is the power?

Algebra Level 1

A , B , C A,B,C are integers satisfying A B C = 2 2 2 . {\large A^{B^{^C}} = 2^{2^{^2}}}. One possible solution is ( A , B , C ) = ( 2 , 2 , 2 ) . (A,B,C) = (2,2,2).

How many other solutions ( A , B , C ) (A,B,C) are there?

2 4 8 Infinitely many

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10 solutions

David Vreken
Aug 28, 2018

Here are a few variations that would yield infinitely many solutions:

A = 16 A = 16 , B = 1 B = 1 , and C = n C = n for any integer n n

A = 16 A = 16 , B = n B = n , and C = 0 C = 0 for any non-zero integer n n

A = 1 6 n m A = 16^{n^m} , B = n B = n , and C = m C = -m for any non-zero integers m m and n n

Moderator note:

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 \large 2^{3+1}

There is an "implicit parentheses" where the addition is grouped together. The 3 + 1 3 + 1 happens first, and then 2 4 2^4 occurs.

In order to be consistent, this grouping has to happen with the exponents as well. With 2 3 4 , 2^{3^4} , the 3 4 3^4 needs to happen first, since it is grouped in the same way as 3 + 1 3+1 in the previous expression.

Yay! I love the last one!

Pi Han Goh - 2 years, 9 months ago

It might just be because it's 1:20am here, but I don't get the last one. Can someone please explain?

Ali Al-Ali - 2 years, 9 months ago

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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 = 16 A^{B^C}=(16^{n^m})^{n^{-m}}=16^{n^m\times n^{-m}}=16^{n^{m-m}}=16^{n^0}=16^1=16

Joseph Newton - 2 years, 9 months ago

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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.

Paul Cockburn - 2 years, 8 months ago

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@Paul Cockburn No, Joseph and mine are the same: A = 1 6 n m A = 16^{n^m} , B = n B = n , and C = m C = -m .

David Vreken - 2 years, 8 months ago

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.

Kevin Higby - 2 years, 9 months ago

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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/

David Vreken - 2 years, 9 months ago

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...

Roland van Vliembergen - 2 years, 9 months ago

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Ahh that clears it up! Thanks!

Kevin Higby - 2 years, 9 months ago

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

Robin Anson - 2 years, 9 months ago

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If n = 2 n = 2 and m = 1 m = 1 for the third option, A = 1 6 n m = 1 6 2 1 = 1 6 2 = 256 A = 16^{n^m} = 16^{2^1} = 16^2 = 256 . Then A B C = 25 6 2 1 = 25 6 1 2 = 16 A^{B^C} = 256^{2^{-1}} = 256^{\frac{1}{2}} = 16 .

David Vreken - 2 years, 9 months ago

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.

Rishi Jayaram - 2 years, 8 months ago

Another scenario that shows there are infinitely many solutions:

A=16, B=0, C=n for any integer n

Rishi Jayaram - 2 years, 8 months ago

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No, because then A B C = 1 6 0 n = 1 6 0 = 1 A^{B^C} = 16^{0^n} = 16^0 = 1 , not 16 16 .

David Vreken - 2 years, 8 months ago

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You are right, David. My oversight!

Rishi Jayaram - 2 years, 8 months ago
Brian Moehring
Aug 27, 2018

For any integer N , N, 1 6 1 N = 2 2 2 , 16^{1^N}=2^{2^2}, which yields the infinitely many \boxed{\text{infinitely many}} solutions given by ( A , B , C ) = ( 16 , 1 , N ) (A,B,C)=(16,1,N)

Nice. There are other general solutions as well. I hope that other people can "find them all"!

Pi Han Goh - 2 years, 9 months ago

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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 . B. I've already done B = 1 , B=1, so there's only infinitely more values of B B remaining to check! Easy peasy!

Brian Moehring - 2 years, 9 months ago

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But we still can find infinitely many solutions where A 16 A\ne 16 .

Pi Han Goh - 2 years, 9 months ago

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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 A is better. Then just work up the tower

Brian Moehring - 2 years, 9 months ago

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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!

Pi Han Goh - 2 years, 9 months ago

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@Pi Han Goh Yes, great question. I wish I could "Like" it, but alas, that is no longer an option. :(

Brian Charlesworth - 2 years, 9 months ago

@Pi Han Goh I'm only finding +/- variations of 2 4 1 \large 2^{4^{1}} and 4 2 1 \large 4^{2^{1}} , in addition to those already mentioned.

Edit: Oh wait, there are powers like 25 6 2 1 , 409 6 3 1 \large 256^{2^{-1}}, 4096^{3^{-1}} , and on up. So yeah, infinitely many .... :)

Brian Charlesworth - 2 years, 9 months ago

2 2 2 = 16 = 1 6 B 0 2^{2^2} = 16 = 16^{B^0} , where B B can be any integer except 0, so long as A = 16 A=16 and C = 0 C=0 . Therefore, there are infinitely many other solutions.

Of course I shoud have seen that!

Barbara Geller - 2 years, 9 months ago
Munem Shahriar
Sep 2, 2018

For ( A , B , C ) = ( 2 20 x , 5 x , 1 ) (A,B,C) = (2^{20x}, 5x, -1)

A B C = ( 2 20 x ) ( 5 x ) 1 = ( 2 20 x ) 1 5 x = 2 20 x × 1 5 x = 2 4 = 2 2 2 \large A^{B^C} = {(2^{20x})}^{(5x)^{-1}} = (2^{20x})^{\frac 1{5x}} = 2^{20x \times \frac 1{5x}} = 2^4 = 2^{2^2}

Where x x can be any non-zero integer. Hence there are infinitely many solutions for ( A , B , C ) (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.

Alan Laifer - 2 years, 9 months ago
N Kansara
Sep 3, 2018

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

Luca Heinemann
Sep 6, 2018

For any non-zero integer n n ,

2 2 2 = 16 = 1 6 n 0 2^{2^2} = 16 = 16^{n^0}

Therefore there are infinitely many solutions.

Kuuaebo Cowou
Sep 3, 2018

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

Des Cayce - 2 years, 9 months ago

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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?

kuuaebo cowou - 2 years, 9 months ago
Jam M
Sep 7, 2018

1 6 B 0 = 2 2 2 16^{B^0} = 2^{2^2} for all nonzero integers B B .

Stary Traper
Sep 5, 2018

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

Ervyn Manuyag
Sep 2, 2018

Bcuz any number can be n

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