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2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8= 2^9 2^10 2^16 2^35 2^37

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Manager
Joined: 25 Feb 2008
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2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8= 2^9 2^10 2^16 2^35 2^37 [#permalink]

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12 Jun 2008, 12:25
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2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8=

2^9
2^10
2^16
2^35
2^37

Kudos [?]: 14 [0], given: 0

CEO
Joined: 17 Nov 2007
Posts: 3584

Kudos [?]: 4584 [0], given: 360

Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40

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12 Jun 2008, 12:35
A

1)
2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8=
2^2+2^2+2^3+2^4+2^5+2^6+2^7+2^8=
2^3+2^3+2^4+2^5+2^6+2^7+2^8=
2^4+2^4+2^5+2^6+2^7+2^8=
2^5+2^5+2^6+2^7+2^8=
2^6+2^6+2^7+2^8=
2^7+2^7+2^8=
2^8+2^8=
2^9

2) use formula: $$1+q^1+q^2+...+q^n=\frac{q^{n+1}-1}{q-1}$$

$$1+(1+2^1+2^2+...+2^8)=1+\frac{2^{9}-1}{2-1}=1+2^9-1=2^9$$
_________________

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Kudos [?]: 4584 [0], given: 360

SVP
Joined: 30 Apr 2008
Posts: 1867

Kudos [?]: 615 [1], given: 32

Location: Oklahoma City
Schools: Hard Knocks

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12 Jun 2008, 12:47
1
KUDOS
The other rule you can know is that when you add two numbers that are the same with the same exponents, the sum = that same number with exponent +1....see below:

$$2^2 + 2^2 = 2^3$$
$$4 + 4 = 8$$

So The first 2 + 2 can be viewed as $$2^1 + 2^1 = 2^2$$, then you add that to another $$2^2$$ to get $$2^3$$ and so on, until you get to $$2^8 + 2^8 = 2^9$$

If I see a question involving exponents and adding, subtracting, multiplying or dividing their bases, etc. i will often try it with something I know the value of easily. Like a base of 2, 3 or 4. Then I take the value of the exponent, do the operation and see if I recognize the result.

Like 2^2 + 2^2 = 8, which is 2^3. So that makes me realize the pattern. $$2^x + 2^x = 2^{x+1}$$

PLEASE NOTE:: This formula really only works for base of 2. If you have base of 3, then you need $$n^x + n^x + n^x = n^{x+1}$$ (See how there are 3 n's? Whatever the base is, you need that number of $$n^x$$'s.
_________________

------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$. GMAT Club Premium Membership - big benefits and savings Last edited by jallenmorris on 13 Jun 2008, 06:42, edited 2 times in total. Kudos [?]: 615 [1], given: 32 Current Student Joined: 28 Dec 2004 Posts: 3351 Kudos [?]: 319 [0], given: 2 Location: New York City Schools: Wharton'11 HBS'12 Re: Weird Sum-Gmat prep [#permalink] Show Tags 12 Jun 2008, 13:30 Capthan wrote: 2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8= 2^9 2^10 2^16 2^35 2^37 please explain the answer. Thanks weird indeed.. here is how i do it.. 2+2=2^2 2^2+2^2=2^3 2^3+2^3=2^4 2^4+2^4=2^5.. u can see the pattern. 2^n + 2^n=2^(N+1)..in our case 2^8..will be 2^9 the ans will be 2^9.. Kudos [?]: 319 [0], given: 2 Director Joined: 01 Jan 2008 Posts: 504 Kudos [?]: 54 [0], given: 0 Re: Weird Sum-Gmat prep [#permalink] Show Tags 12 Jun 2008, 13:51 Although this approach may not suit everyone, it is easy to realise that the question is Geometric Progression, and the formula for a sum of a GP can be used to get the answer. The answer is 2^9 Kudos [?]: 54 [0], given: 0 Manager Joined: 11 Apr 2008 Posts: 128 Kudos [?]: 59 [0], given: 0 Location: Chicago Re: Weird Sum-Gmat prep [#permalink] Show Tags 12 Jun 2008, 14:04 jallenmorris wrote: The other rule you can know is that when you add two numbers that are the same with the same exponents, the sum = that same number with exponent +1....see below: $$2^2 + 2^2 = 2^3$$ $$4 + 4 = 8$$ So The first 2 + 2 can be viewed as $$2^1 + 2^1 = 2^2$$, then you add that to another $$2^2$$ to get $$2^3$$ and so on, until you get to $$2^8 + 2^8 = 2^9$$ If I see a question involving exponents and adding, subtracting, multiplying or dividing their bases, etc. i will often try it with something I know the value of easily. Like a base of 2, 3 or 4. Then I take the value of the exponent, do the operation and see if I recognize the result. Like 2^2 + 2^2 = 8, which is 2^3. So that makes me realize the pattern. $$n^x + n^x = n^{x+1}$$ The formula $$n^x + n^x = n^{x+1}$$ does not work for all numbers though. For example $$3^2 + 3^2$$ does not equal $$3^3$$. Does this formula only apply to a base of 2? _________________ Factorials were someone's attempt to make math look exciting!!! Kudos [?]: 59 [0], given: 0 Senior Manager Joined: 29 Aug 2005 Posts: 272 Kudos [?]: 69 [0], given: 0 Re: Weird Sum-Gmat prep [#permalink] Show Tags 12 Jun 2008, 23:22 Capthan wrote: 2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8= 2^9 2^10 2^16 2^35 2^37 please explain the answer. Thanks 2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8= 2+2+4+8+16+32+64+128+256 512 or 2^9 so A should be the answer _________________ The world is continuous, but the mind is discrete Kudos [?]: 69 [0], given: 0 Director Joined: 27 May 2008 Posts: 541 Kudos [?]: 363 [0], given: 0 Re: Weird Sum-Gmat prep [#permalink] Show Tags 12 Jun 2008, 23:49 another way of solving this. S = 2 + 2 + 2^2 + 2^3 .......... + 2^8 -------- EQ1 Multiply 2 on both sides 2S = 2*2 + 2^2 + 2^3 + 2^4 ......... + 2^9 ----------- EQ2 EQ2 - EQ1 2S - S = 2*2 + 2^9 - (2+2) (all other terms will get canclled) S = 2^9 Kudos [?]: 363 [0], given: 0 SVP Joined: 30 Apr 2008 Posts: 1867 Kudos [?]: 615 [0], given: 32 Location: Oklahoma City Schools: Hard Knocks Re: Weird Sum-Gmat prep [#permalink] Show Tags 13 Jun 2008, 06:35 I just realized that this morning when working on another problem. It works if the base is 2. Essentially, you have to have the same number of numbers with exponents that are the same in order to combine them all with the same base and increase the exponent by 1. $$4^4 + 4^4 + 4^4 + 4^4 = 4^5$$ $$5^2 + 5^2 + 5^2 + 5^2 + 5^2 = 5^3$$ I made the mistake of taking something that works for 2 and applying it to others. That doesn't work as often as I wish it did brokerbevo wrote: jallenmorris wrote: The other rule you can know is that when you add two numbers that are the same with the same exponents, the sum = that same number with exponent +1....see below: $$2^2 + 2^2 = 2^3$$ $$4 + 4 = 8$$ So The first 2 + 2 can be viewed as $$2^1 + 2^1 = 2^2$$, then you add that to another $$2^2$$ to get $$2^3$$ and so on, until you get to $$2^8 + 2^8 = 2^9$$ If I see a question involving exponents and adding, subtracting, multiplying or dividing their bases, etc. i will often try it with something I know the value of easily. Like a base of 2, 3 or 4. Then I take the value of the exponent, do the operation and see if I recognize the result. Like 2^2 + 2^2 = 8, which is 2^3. So that makes me realize the pattern. $$n^x + n^x = n^{x+1}$$ The formula $$n^x + n^x = n^{x+1}$$ does not work for all numbers though. For example $$3^2 + 3^2$$ does not equal $$3^3$$. Does this formula only apply to a base of 2? _________________ ------------------------------------ J Allen Morris **I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

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Kudos [?]: 615 [0], given: 32

CEO
Joined: 29 Mar 2007
Posts: 2554

Kudos [?]: 515 [0], given: 0

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13 Jun 2008, 06:55
Capthan wrote:
2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8=

2^9
2^10
2^16
2^35
2^37

2+2 -> 2(2) = 2^2+2^2 --> 2(2^2) --> 2^3 +2^3 --> 2(2^3) etc...

2^9

A

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Director
Joined: 14 Aug 2007
Posts: 726

Kudos [?]: 212 [0], given: 0

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13 Jun 2008, 07:19
2^9, by same method mentioned by walker

walker wrote:
A

1)
2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8=
2^2+2^2+2^3+2^4+2^5+2^6+2^7+2^8=
2^3+2^3+2^4+2^5+2^6+2^7+2^8=
2^4+2^4+2^5+2^6+2^7+2^8=
2^5+2^5+2^6+2^7+2^8=
2^6+2^6+2^7+2^8=
2^7+2^7+2^8=
2^8+2^8=
2^9

Kudos [?]: 212 [0], given: 0

Re: Weird Sum-Gmat prep   [#permalink] 13 Jun 2008, 07:19
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2+2+2^2+2^3+2^4+2^5+2^6+2^7+2^8= 2^9 2^10 2^16 2^35 2^37

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