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Ax(y) is an operation that adds 1 to y

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Ax(y) is an operation that adds 1 to y [#permalink] New post 03 Jul 2012, 17:17
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Question Stats:

34% (03:08) correct 66% (02:33) wrong based on 162 sessions
Ax(y) is an operation that adds 1 to y and then multiplies the result by x. If x = 2/3, then Ax(Ax(Ax(Ax(Ax(x))))) is between

(A) 0 and ½
(B) ½ and 1
(C) 1 and 1½
(D) 1½ and 2
(E) 2 and 2½
[Reveal] Spoiler: OA
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Re: Ax(y) is an operation that adds 1 to y [#permalink] New post 03 Jul 2012, 20:51
If I read the question correctly, AX(Y) = (1+Y)X
Putting X=2/3,
=> AX(X) = (1+ 2/3)*(2/3)
=> AX(X) = (5/3)*(2/3)
=> AX(X) = 10/9 > 1

AX(AX(X) = (1+10/9)(10/9)....The first term in the bracket will be greater than 2 and next term is greater than one...so the product is greater than 2.
The next operation will make the (1+Y) term greater than 3, so basically as per my understanding there is some information missing in the question or I misunderstood.

Regards,
Tushar
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Re: Ax(y) is an operation that adds 1 to y [#permalink] New post 04 Jul 2012, 01:32
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Smita04 wrote:
Ax(y) is an operation that adds 1 to y and then multiplies the result by x. If x = 2/3, then Ax(Ax(Ax(Ax(Ax(x))))) is between

(A) 0 and ½
(B) ½ and 1
(C) 1 and 1½
(D) 1½ and 2
(E) 2 and 2½

Sorry, but I don't have the OA.


The question should read:

A_x(y) is an operation that adds 1 to y and then multiplies the result by x. If x = 2/3, then A_x(A_x(A_x(A_x(A_x(x))))) is between

(A) 0 and ½
(B) ½ and 1
(C) 1 and 1½
(D) 1½ and 2
(E) 2 and 2½

According to the stem:
A_x(x)=(x+1)x=x^2+x;
A_x(A_x(x))=A_x(x^2+x)=(x^2+x+1)*x=x^3+x^2+x;
A_x(A_x(A_x(x)))=A_x(x^3+x^2+x)=(x^3+x^2+x+1)*x=x^4+x^3+x^2+x;
...

We can see the pattern now, so A_x(A_x(A_x(A_x(A_x(x)))))=x^6+x^5+x^4+x^3+x^2+x.

For x=\frac{2}{3} we'll get: (\frac{2}{3})^6+(\frac{2}{3})^5+(\frac{2}{3})^4+(\frac{2}{3})^3+(\frac{2}{3})^2+(\frac{2}{3}).

So, we have the sum of the 6 terms of the geometric progression with the first term equal to \frac{2}{3} and the common ratio also equal to \frac{2}{3}.

Now, the sum of infinite geometric progression with common ratio |r|<1, is sum=\frac{b}{1-r}, where b is the first term. So, if we had infinite geometric progression instead of just 6 terms then its sum would be Sum=\frac{\frac{2}{3}}{1-\frac{2}{3}}=2. Which means that the sum of this sequence will never exceed 2, also as we have big enough number of terms (6) then the sum will be very close to 2, so we can safely choose answer choice D.

Answer: D.

One can also use direct formula.
We have geometric progression with b=\frac{2}{3}, r=\frac{2}{3} and n=6;

S_n=\frac{b(1-r^n)}{(1-r)} --> S_{6}=\frac{\frac{2}{3}(1-(\frac{2}{3})^{6})}{(1-\frac{2}{3})}=2*(1-(\frac{2}{3})^{6}). Since (\frac{2}{3})^{6} is very small number then 1-(\frac{2}{3})^{6} will be less than 1 but very close to it, hence 2*(1-(\frac{2}{3})^{6}) will be less than 2 but very close to it.

Answer: D.

Hope it's clear.
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Re: Ax(y) is an operation that adds 1 to y [#permalink] New post 21 Jun 2013, 02:47
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Re: Ax(y) is an operation that adds 1 to y [#permalink] New post 22 Jun 2013, 01:27
Smita04 wrote:
Ax(y) is an operation that adds 1 to y and then multiplies the result by x. If x = 2/3, then Ax(Ax(Ax(Ax(Ax(x))))) is between

(A) 0 and ½
(B) ½ and 1
(C) 1 and 1½
(D) 1½ and 2
(E) 2 and 2½


This one was quite tricky.. lovely explanation by Bunuel using geometric progression.
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Re: Ax(y) is an operation that adds 1 to y [#permalink] New post 15 Jan 2014, 18:53
tusharacc wrote:
If I read the question correctly, AX(Y) = (1+Y)X
Putting X=2/3,
=> AX(X) = (1+ 2/3)*(2/3)
=> AX(X) = (5/3)*(2/3)
=> AX(X) = 10/9 > 1

AX(AX(X) = (1+10/9)(10/9)....The first term in the bracket will be greater than 2 and next term is greater than one...so the product is greater than 2.
The next operation will make the (1+Y) term greater than 3, so basically as per my understanding there is some information missing in the question or I misunderstood.

Regards,
Tushar



Hi Bunnel,

Please explain what is wrong in the above method?
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Re: Ax(y) is an operation that adds 1 to y [#permalink] New post 15 Jan 2014, 20:56
Expert's post
tusharacc wrote:
If I read the question correctly, AX(Y) = (1+Y)X
Putting X=2/3,
=> AX(X) = (1+ 2/3)*(2/3)
=> AX(X) = (5/3)*(2/3)
=> AX(X) = 10/9 > 1

AX(AX(X) = (1+10/9)(10/9)....The first term in the bracket will be greater than 2 and next term is greater than one...so the product is greater than 2.
The next operation will make the (1+Y) term greater than 3, so basically as per my understanding there is some information missing in the question or I misunderstood.

Regards,
Tushar


The highlighted portion is incorrect here.

Ax(Ax(X)) = (1+10/9)(2/3)
Note that X is 2/3 only. Ax(X) is 10/9.
Ax(10/9) = (1 + 10/9)(2/3)
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Re: Ax(y) is an operation that adds 1 to y   [#permalink] 15 Jan 2014, 20:56
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