Lay out the base and powers

Algebra Level 1

If A B = 8 2 , A^B = 8^2, is it true that A × B A\times B must equal 16?

Yes No

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24 solutions

Chew-Seong Cheong
Aug 26, 2018

The answer is " No " because digital solutions alone we have three distinct solutions: A B = 8 2 = 2 6 = 4 3 = 6 4 1 A × B = { 8 × 2 = 16 2 × 6 = 12 4 × 3 = 12 64 × 1 = 64 A^B = 8^2 = 2^6 = 4^3 =64^1 \implies A\times B = \begin{cases} 8 \times 2 = 16 \\ 2 \times 6 = 12 \\ 4 \times 3 = 12 \\ 64 \times 1 = 64 \end{cases}

Can we write unreal,irrational & rational solutions?

Gia Hoàng Phạm - 2 years, 9 months ago

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Yes, of course. Just for real A B = 8 2 B ln A = 6 ln 2 ln A = 6 ln 2 B A^B = 8^2 \implies B \ln A = 6 \ln 2 \implies \ln A = \dfrac {6 \ln 2}B . Implying for B ( 0 , ) B \in (0, \infty) , A ( , ) A \in (-\infty, \infty) . There are infinitely many solutions. If B < 0 B< 0 , there will be complex solutions for A A .

Chew-Seong Cheong - 2 years, 9 months ago

the question wasn't clear!

Mauricio Belmont - 2 years, 9 months ago

Or it could just be 64¼½ which is 8. 8 squared is 64, but 8 × 2 is 16 - which is AB, which must either be 4 squared or 8 x 2.

Kieran Halfpenny - 2 years, 9 months ago

8^2=8^2 so then A^B=8^2 A×B=8×2=16

Lohith Kumar - 2 years, 9 months ago

This is questionable. A^B=8^2=2^6= B^C this is what I read... there’s not written A^B=64

Federico Z - 2 years, 9 months ago

Does it mean to say that this question has finite solution.

A Former Brilliant Member - 2 years, 9 months ago

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Infinite many solutions. Just for real solutions A B = 8 2 B ln A = 6 ln 2 ln A = 6 ln 2 B A^B = 8^2 \implies B \ln A = 6 \ln 2 \implies \ln A = \dfrac {6 \ln 2}B . Implying for B ( 0 , ) B \in (0, \infty) , A ( , ) A \in (-\infty, \infty) . There are infinitely many solutions. If B < 0 B< 0 , there will be complex solutions for A A .

Chew-Seong Cheong - 2 years, 9 months ago

Eh, you assign 8 to A and 2 to B right? I was always told that variables in math are immutable, so how can A become 2 and B become 6 then?

Hans Isbrücker - 2 years, 9 months ago

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A B = 8 2 = ( 2 3 ) 2 = 2 6 A^B = 8^2 = (2^3)^2 = 2^6 . I don't understand the immutable bit.

Chew-Seong Cheong - 2 years, 9 months ago
Joseph Newton
Aug 20, 2018

Integer solutions include ( A , B ) = ( 8 , 2 ) , ( 4 , 3 ) , ( 2 , 6 ) , ( 1 , 64 ) (A,B)=(8,2),(4,3),(2,6),(1,64) , giving A × B = 16 , 12 , 12 , 64 A\times B=16,12,12,64 . However there are many more solutions if one considers that A A and B B don't necessarily have to be integers: A B = 64 ln A B = ln 64 B ln A = ln 64 B = ln 64 ln A A × B = A ln A ln 64 f ( A ) = A × B is a continuous function for A > 1 When A = 3 , A × B 11.3567 When A = 13 , A × B 21.0786 (just any larger number will do for this proof.) By the intermediate value theorem, there exists real values of A such that A × B = 12 , 13 , 14 , etc. \begin{aligned}A^B&=64\\\ln A^B&=\ln64\\B\ln A&=\ln64\\B&=\frac{\ln64}{\ln A}\\A\times B&=\frac A{\ln A}\ln64\\f(A)=A\times B&\text{ is a continuous function for }A>1\\ \text{When }A=3,&\,A\times B\approx11.3567\\\text{When }A=13,&\,A\times B\approx21.0786\ \text{(just any larger number will do for this proof.)}\\ \text{By the intermediate value theorem, }&\text{there exists real values of }A\text{ such that }A\times B=12,13,14,\text{ etc.}\end{aligned}

What about 64^1?

Ray C - 2 years, 9 months ago

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The problem has changed since I posted this solution, and originally asked for whether 12, 16 or 20 are possible, to which the answer is all of them. Getting 20 as an answer requires A and B to be irrational, which I suppose is now the key point in this solution. I have edited the solution to reflect this.

Joseph Newton - 2 years, 9 months ago

Great proof

Shay Lempert - 2 years, 9 months ago
Ram Mohith
Aug 21, 2018

A B = 6 4 1 = 8 2 ( o r ) ( 8 ) 2 = 2 6 ( o r ) ( 2 ) 6 = 4 3 \large A^B = {\color{#D61F06}64^1} = {\color{#3D99F6}8^2} \quad (or) \quad {\color{#3D99F6}(-8)^2} = \quad {\color{#E81990}2^6} \quad (or) \quad {\color{#E81990}(-2)^6} = \quad {\color{#20A900}4^3}

So, the possible values of A × B A \times B are : 64 × 1 = 64 8 × 2 = 16 ( o r ) 8 × 2 = 16 2 × 6 = 12 ( o r ) 2 × 6 = 12 4 × 3 = 12 \begin{array}{c}~\color{#D61F06} 64 \times 1 = 64 \\ \color{#3D99F6} 8 \times 2 = 16 \quad {\color{#333333}(or)} \quad -8 \times 2 = -16 \\ \color{#E81990} 2 \times 6 = 12 \quad {\color{#333333}(or)} \quad -2 \times 6 = -12 \\ \color{#20A900} 4 \times 3 = 12 \\ \end{array}

Since we now have very similar solutions, I deleted my solution and upvoted yours.

David Vreken - 2 years, 9 months ago

A and B are not necessarily integers...

Canwen Jiao - 2 years, 9 months ago

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smart.....

Kaisei Tanaka - 2 years, 9 months ago
Jeremy Galvagni
Aug 26, 2018

The red graph is ( A , B ) (A,B) such that A B = 64 A^{B}=64 . Although the point ( 8 , 2 ) (8,2) can be seen, there are plenty of other points.

A B = 64 A^{B}=64 means B = ln 64 ln A B=\frac{\ln{64}}{\ln{A}} . So A × B = A ln 64 ln A A\times B = A\frac{\ln{64}}{\ln{A}} . This is the black graph.

Again, directly above ( 8 , 2 ) (8,2) is ( 8 , 16 ) (8,16) but the black graph reaches plenty of other heights.

In fact every height e ln 64 e\ln{64} or above and below 0 0 is in the range.

Alex Wang
Aug 26, 2018

64^1 works

This is all I needed to do for this question, too. No need to dive further unless there was a bonus question for a more general case

Sean Demers - 2 years, 9 months ago

Or (the fifth root of 64)^5, since 64 is not a perfect 5th

alex wang - 2 years, 9 months ago

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"64 is not a perfect cube"? Then what is a perfect cube?! o.O

C . - 2 years, 9 months ago

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Oh wait im an idiot. Facepalm.

alex wang - 2 years, 9 months ago
Haoran Wang
Aug 26, 2018

well... 409 6 1 2 4096^{\frac{1}{2}} works, and product of A and B is 2048.

Pepper Mint
Aug 27, 2018

A = 2 2 i A=2\sqrt{2}i and B = 4 B=4 . There was no statement that A A and B B must be real numbers.

The question being asked is must be true, what you have shown here is that it may be true.

Bernard Zimmermann - 2 years, 9 months ago

Since A B = 8 2 = 64 A^{B} = 8^{2} = 64 we have:

B = l o g A 64 B = log_{A}{64}

And, therefore:

A B = A l o g A 64 = l o g A 6 4 A AB = A\cdot log_{A}{64} = log_{A}{64^{A}}

The latter relation being independent of the value of A A , and thus valid for each real value of A A and B B . We could, for example, give to A A and B B the values:

A = e A = e

B = l n 64 4.159 B = ln{64} \approx 4.159

Or trivially:

A = 64 A = 64

B = 1 B = 1

In both cases, A B 16 A \cdot B \neq 16 , being approximately 11.305 11.305 if A = e A = e and 64 64 if A = 64 A = 64 .

Simon The Great
Aug 27, 2018

what we know: A^B = 8^2 = 64

what we want to know: Does AxB always = 16?

64^1 also equals 64 so no

Sergio España
Sep 1, 2018

Hello, 4², Bye

Davide Guerri
Sep 1, 2018

No. For instance: 2^6 = 8^2 so A x B could also be 12 (4^3 is another one)

Agata Pokorska
Aug 31, 2018

The answer is "No" because (-8)^2 = 8^2 so A may equals -8 or 8. If A equals -8, A*B = -16.

Robert Vandor
Aug 31, 2018

8x2=16 2^4=16

Andrew Williams
Aug 28, 2018

The awnser must be NO , since: (b)^a =(-b)^a BUT b x a Does not equal -b x a

In this case, 8^2 is equal to (-8)^2, both equal to 64, but the first expression is equal to 64 while the latter is -64

We could provide the easy counterexample:

A B = 8 2 A = 16 B A^B = 8^2 \Leftrightarrow A = \sqrt[B]{16}

For example B : = 3 B:=3 , then:

A = 16 3 A=\sqrt[3]{16}

and obviously

A B = 16 3 3 16 A^B = \sqrt[3]{16} \cdot 3 \neq 16

Gia Hoàng Phạm
Aug 28, 2018

Because 64 64 can write in form of A B = 2 6 = 4 3 = 8 2 = 6 4 1 = A^B=2^6=4^3=8^2=64^1=\dots which the answer(s) is { 8 × 2 = 16 2 × 6 = 12 4 × 3 = 12 64 × 1 = 64 = \begin{cases} 8 \times 2=16 \\ 2 \times 6=12 \\ 4 \times 3=12 \\ 64 \times 1=64 \\ =\dots \end{cases} so the answer is N o \boxed{\large{No}}

Benjamin Chen
Aug 27, 2018

For the sake of putting π \pi in every solution : A = 6 4 1 π A = 64^\frac{1}{\pi} and B = π B = \pi which gives us A × B = 6 4 1 π × π \boxed{A \times B = 64^\frac{1}{\pi}\times \pi } (Irrationnal number)

Ido Wiseman
Aug 27, 2018

6 4 1 = 8 2 64^1 = 8^2

64 × 1 16 64 \times 1 \neq 16

Kevin Long
Aug 27, 2018

Since 2^6=64 2*6=12 12 does not equal 16.

Gernot Niedoba
Aug 27, 2018

(-8)^2 = 8^2 and gives as A x B.......-16

8^2=(2^3)^2 =2^6 So A=2 , B=6 here which equals 12 not 16 so it's not true

Amit Kumar
Aug 26, 2018

A * B should be 64

Zain Majumder
Aug 26, 2018

A B = 8 2 = 64 B = log A 64 A^B = 8^2 = 64 \implies B = \log_A64

A × B = A log A 64 A \times B = A\log_A64 . Plug in almost any value for A A and the result will not be 16 16 . For example, A = 2 A=2 yields A × B = 12 A \times B = 12 . Also, if 0 < A < 1 0 < A < 1 , the answer will always be negative and therefore will never be 16 16 .

Kelvin Hong
Aug 26, 2018

Since A B = 8 2 = 4 3 = 2 6 A^B=8^2=4^3=2^6

The possible value of A B AB is 16 , 12 16,12 . This is enough to raise a contradiction, hence no.

This question should state that A A and B B are Integers so that it is more challenging.

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