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# If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is

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If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is [#permalink]

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23 Jul 2014, 21:35
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49% (01:47) correct 51% (01:16) wrong based on 167 sessions

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If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is the value of the infinite sum 1/2^1+2/2^2+3/2^3+4/2^4+....?

A) 1
B) 2
C) 3
D) π
E) Infinite
[Reveal] Spoiler: OA

Last edited by maggie27 on 02 Aug 2014, 13:57, edited 1 time in total.

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Re: If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is [#permalink]

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23 Jul 2014, 23:04
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maggie27 wrote:
If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is the value of the infinite sum 1/2^1+2/2^22+3/2^3+4/2^4+....?

A) 1
B) 2
C) 3
D) π
E) Infinite

Sum $$= 1/2^1 + 1/2^2 + 1/2^3+1/2^4+... = 1$$
Sum = 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + ... = 1 ..............(I)

Note that the first term is 1/2 so the sum of the rest of the terms must be 1/2 to add up to a total of 1.
So 1/4 + 1/8 + 1/16 + 1/32 + ... = 1/2

Similarly, now the first term is 1/4 so the sum of the rest of the terms must be 1/4 too to get a sum of 1/2.

and so on...

Required sum $$= \frac{1}{2^1}+\frac{2}{2^2}+\frac{3}{2^3}+\frac{4}{2^4}+....$$

Required sum $$=(\frac{1}{2^1}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...) + (\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...) + (\frac{1}{2^3}+\frac{1}{2^4}+...) + ...$$

Required sum $$= (1) + (\frac{1}{2}) + (\frac{1}{4}) + (\frac{1}{8})+ ...$$

From equation (I) above, we know that 1/2 + 1/4 + 1/8 + ... = 1
So Required sum = 1 + 1 = 2
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Re: If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is [#permalink]

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23 Jul 2014, 23:08
maggie27 wrote:
If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is the value of the infinite sum 1/2^1+2/2^22+3/2^3+4/2^4+....?

A) 1
B) 2
C) 3
D) π
E) Infinite

A crude method

1/2^1+2/2^22+3/2^3+4/2^4+....n/2^n

1/2+1/2+3/8+4/16+5/32+6/64+7/128+8/512+...

1+0.375+0.25+0.15+0.09+0.04+0.015... <2

So infinite sum should equal 2 as n increases 2^n increases exponentially.
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Status: On a mountain of skulls, in the castle of pain, I sit on a throne of blood.
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If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1 [#permalink]

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05 Sep 2014, 03:10
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If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is the value of the infinite sum 1/2^1+2/2^2+3/2^3+4/2^4+....?
A. 1
B. 2
C. 3
D. π
E. Infinite

Intuitive, short and uncomplicated explanation please.

Last edited by AmoyV on 05 Sep 2014, 03:39, edited 1 time in total.

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Re: If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1 [#permalink]

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05 Sep 2014, 03:37
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AmoyV wrote:
If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is the value of the infinite sum 1/2^1+2/2^2+3/2^3+4/2^4+....?
A. 1
B. 2
C. 3
D. π
E. Infinite

$$\frac{1}{2^1}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...=1$$

Now, $$\frac{1}{2^1}+\frac{2}{2^2}+\frac{3}{2^3}+\frac{4}{2^4}+...$$ can be written as

$$(\frac{1}{2^1}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...) + (\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+\frac{1}{2^5}+...) + (\frac{1}{2^3}+\frac{1}{2^4}+\frac{1}{2^5}+\frac{1}{2^6}+...) +...$$

$$= 1+ [(\frac{1}{2^1}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...) - \frac{1}{2^1}] + [(\frac{1}{2^1}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...) - \frac{1}{2^1} - \frac{1}{2^2}] +...$$

$$= 1+ (1 - \frac{1}{2}) + (1 - \frac{1}{2} - \frac{1}{4}) +...$$

$$= 1+ \frac{1}{2} + \frac{1}{4} +... = 1 + 1 = 2$$

Hope that helps.
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Re: If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1 [#permalink]

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05 Sep 2014, 06:03
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Let
S= 1/2+2/2^2+3/2^3.......

- S/2= 1/2^2+2/2^3........ dividing both sides by half and subtracting)

S/2 = 1/2+1/2^2+1/2^3.....(This R.H.S is given to be 1 in question)

So
S/2=1
S=2

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Re: If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is [#permalink]

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03 Dec 2017, 22:56
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Re: If the infinite sum 1/2^1+1/2^2+1/2^3+1/2^4+...=1, what is   [#permalink] 03 Dec 2017, 22:56
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