Powers of 2

Algebra Level 1

2 n + 2 n 1 2 n + 1 2 n = ? \large \frac{2^n+2^{n-1}}{2^{n+1}-2^n} = \, ?

1 2 \frac12 1 1 3 2 \frac32 2 2

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10 solutions

Anuj Shikarkhane
Jun 3, 2017

Relevant wiki: Rules of Exponents - Algebraic

This expression can be simplified as

2 n ( 1 + 2 1 ) 2 n ( 2 1 1 ) \large\dfrac{2^n(1 + 2^{-1})}{2^n(2^1 - 1)}

= 1 + 1 2 1 \large\dfrac{1+\dfrac{1}{2}}{1}

= 3 2 \large\boxed{\dfrac{3}{2}}

They are asking power of 2 in the answer.

SATPAL SINGH - 4 years ago

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"Powers of 2" is just the title/'nickname' for the question. No way is it related to what the problem statement is.

Shourya Pandey - 4 years ago

How can i post questions in this community forum.

SATPAL SINGH - 4 years ago

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This is too much easy

Razwan Al-Wasi - 3 years, 11 months ago

This question is from R.D. Sharma - A book of Ninth class in India.

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Is it? I didn't realise (and I've finished that whole book). :P

Sharky Kesa - 3 years, 12 months ago

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Really, that's taught in our school. Definitely that is not even close to Brilliant level 2.

Vishwash Kumar ΓΞΩ - 3 years, 12 months ago

you could also substitute n with 2

kavita nair - 3 years, 11 months ago

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Yes, but that is not a mathematical way to solve.

Anuj Shikarkhane - 3 years, 11 months ago

Relevant wiki: Rules of Exponents

2 n + 2 n 1 2 n + 1 2 n = 2 n + 2 n 2 2 n ( 2 ) 2 n = 2 n ( 1 + 1 2 ) 2 n ( 2 1 ) = 3 2 1 = 3 2 = \dfrac{2^n+2^{n-1}}{2^{n+1}-2^n}=\dfrac{2^n+\frac{2^n}{2}}{2^n(2)-2^n}=\dfrac{2^n(1+\frac{1}{2})}{2^n(2-1)}=\dfrac{\frac{3}{2}}{1}=\dfrac{3}{2}= 1.5 \huge\boxed{\color{#D61F06}1.5} a n s w e r \color{#3D99F6}\large \boxed{answer}

Yes, the key here is to see that 2 n 1 = 2 n × 2 1 = 2 n × 1 2 2^{n-1} = 2^n \times 2^{-1} = 2^n \times \frac12

Agnishom Chattopadhyay - 3 years, 11 months ago

Thanks Marvin, your explanation doesn't skip any steps and is clear. Much appreciated :)

Frank F - 3 years, 11 months ago

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You're welcome.

A Former Brilliant Member - 3 years, 11 months ago

By subbing in 1 for n or any number for that matter we will get 3/2

thnx, made it easier:)

Bless up - 3 years, 11 months ago

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(: awesome!

Joseph Gomes Short Films - 3 years, 11 months ago

Exactly how I solved it!

Elsa Wiener - 3 years, 11 months ago

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Nice! we think alike (:

Joseph Gomes Short Films - 3 years, 11 months ago

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Yep, lazyness is universal! :)

Martin Ramsch - 3 years, 4 months ago

Such a substition is a good sense check, but it isn't a mathematically acceptable answer ... it doesn't prove the answer is 3/2 for all values of n.

John Woods - 3 years, 11 months ago

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Which values? Im too lazy to find out myself...

Joseph Gomes Short Films - 3 years, 10 months ago

Nond Nachanon
Jun 19, 2017

Let 2 n 2^n =P

P + P / 2 2 P P \frac{P+P/2}{2P-P}

= 2 P + P 2 ( 2 P P ) \frac{2P+P}{2(2P-P)}

= 2 P + P 4 P 2 P \frac{2P+P}{4P-2P}

= 3 P 2 P \frac{3P}{2P}

Cut P AND P

= 3 2 \frac{3}{2}

Nicely done. By substituting 2^n = P, it makes the expression so much easier to work with...

Pi Han Goh - 3 years, 11 months ago
Daniel Cortild
Jun 5, 2017

We know that 2 n + 1 2 n = 2 n 2^{n+1}-2^n=2^n . Thus 2 n + 2 n 1 2 n + 1 2 n = 2 n + 2 n 1 2 n = 1 + 1 2 = 3 2 = 1.5 \dfrac{2^n+2^{n-1}}{2^{n+1}-2^n}=\dfrac{2^n+2^{n-1}}{2^n}=1+\dfrac12=\dfrac32=1.5

But the question is what will b the power. of 2 .

SATPAL SINGH - 4 years ago

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No, the title says that only because powers of 2 are involved. The answer should not be related to the title unless and until strictly specified by the person posting the problem in his/her problem statement.

Tapas Mazumdar - 4 years ago
Colin Saxelby
Jun 25, 2017

The correct answer is 1.5 of course, but unusually the problem as posed says n is a <real> number, not the usual <integer>. This shouldn't make any difference, but I guess if you got really sneaky and started mixing up the +ve/-ve sign of sqrt(2) (n=0.5) you could conceivably get something else. By letting n=0.333... you could even make it complex, but you'd be doing some socially unacceptable things with the maths.

Taking any no. in the place of n

You have only shown that the answer is 1.5 for some particular value(s) of n. How do you know that there isn't a value of n that doesn't give the answer of 1.5?

Pi Han Goh - 3 years, 11 months ago
Shou Seiyo
Jun 22, 2017

Try with n=1, and you're done... Sure, in that way you cannot be sure the expression is going to simplify "every time" to 3/2, but taking that for granted... This would have been more an interesting question in case the author would have chosen to make an "input a value" type question.

I did make it that. It was changed so it would filter out people who may be under some misconceptions.

Sharky Kesa - 3 years, 11 months ago

Use n=1 as you say and get 2/3, then assume it's true for n=k (some random positive integer), then looking at k+1 we could easily factor out 2 from every term top and bottom and get back to n=k ( you could show the work if you wanted). Therefore, if it's true for some positive integer k, it is also true for k+1. It is calculated to be true for n=1 so it must be true for n=2 and then n=3 ... and so by induction is true for all positive integers n

Mike Hawke - 3 years, 11 months ago
Vk Stonecold
Jun 16, 2017

Take 2 raise to power n common

Yes, I tried that and I got 2 n ( 1 + 2 1 ) 2 n ( 2 1 ) \frac{2^n \left ( 1 + 2^{-1} \right )}{2^n \left ( 2 - 1 \right ) }

Agnishom Chattopadhyay - 3 years, 11 months ago

Common? What do yo mean by that?

Pi Han Goh - 3 years, 11 months ago

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