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If the sum of all \(x^n\), where x is a positive two-digit integer and n is an integer from 0 to 5 inclusive, equals k, which of the following equals x^6?

A. kx + 1

B. k(x - 1) + 1

C. k(x - 1) - 1

D. k(x - 2) + 1

E. k(x - 2) - 1

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\(x^0 + x^1 + x^2 + x^3 + x^4 + x^5 = k\) --- (1)

Multiplying by x on both sides we get

\(x(x^0 + x^1 + x^2 + x^3 + x^4 + x^5) = kx\)

\(x^1 + x^2 + x^3 + x^4 + x^5 + x^6 = kx\)

Adding 1 on both sides

\((1+ x^1 + x^2 + x^3 + x^4 + x^5) + x^6 = kx + 1\)

\(x^0 = 1\)

\((x^0+ x^1 + x^2 + x^3 + x^4 + x^5) + x^6 = kx + 1\)

Substituting the value of equation (1), we get

\((k) + x^6 = kx + 1\)

\( x^6 = kx + 1 - k\)

\( x^6 = k(x-1) + 1\)

Option B
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\(x^0+x^1+x^2...+x^6\)=\((x^6-1)/(x-1)\)

\((x^6-1)=k(x-1)\\
=>x^6=kx-k+1\)
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Assume x = 11 and n = (1,2,3). Therefore k = 11 + 121 + 1331 = 1463
x^4= 14641

If you observe this is equal to 1463 * 10 + 11 which is equal to k * (x-1)+ x
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Bunuel
Try solving by straightforward algebraic manipulation, avoiding overthinking and overcomplicating.
­Hi Bunuel
I have ran into multiple exercises level 700+ that seem to require the same type of logic, like the one used on this exercise.
I am refering specifically to the realization that one needs to come up with a algebraic approach envolving adding numbers that add to 0 (+1; -1; or similar) to an equation.
When do you know you need to apply this kind of logic in an exercise? Do you have an answer for this?

Thanks.
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Approach using the geometric series sum formula

Sn = a * (r^n - 1) / (r-1)

a = 1 (since x^0 is 1)
r = x
n = 6 (x^ 0 to x^5)
Sn = K

K = (1 * (x^6 - 1)) / (x - 1). Now find x^6

x^6 = k(x-1) + 1
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I used a straightforward numerical approach to solve this. I chose the simplest two-digit number, 10.
Adding up its powers from 10^0 to 10^5 gives:

1+10+100+1000+10000+100000=111111=k

Since we need 10^6, I checked the answer choices. At first glance, option B stood out.
Using the given formula:

k(x−1)+1= 111111(10−9)+1=10^6

Thus, option B is the correct answer.
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thank you for posting great variety types of quesstions.
is this level of questions are there in real exam !!
or it for practice purpose.
Bunuel
Try solving by straightforward algebraic manipulation, avoiding overthinking and overcomplicating.
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prathamesh1_0
thank you for posting great variety types of quesstions.
is this level of questions are there in real exam !!
or it for practice purpose.
Bunuel
Try solving by straightforward algebraic manipulation, avoiding overthinking and overcomplicating.

Yes, this question is absolutely up to GMAT standards—both in terms of content and difficulty.
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