Exponents-

3 + 2x3^1 + 2 X 3^2 + 2 X 3^3 + 2 X 3^4 + 2 X 3^5 + 2 X 3^6 + 2 X 3^7

3 + 2 ( 3^1 + 3^2 .... 3^7)

3 + 2 ( 3(3^7 - 1)/2) [using GP Sum formula]

3 + 3^8 - 3

3^8

Dint get the solution if u take 3+2 as common wnt u keep 1 in its position (within the bracket)? why haven't u added 1?

theCodeToGMAT wrote:3 + 2x3^1 + 2 X 3^2 + 2 X 3^3 + 2 X 3^4 + 2 X 3^5 + 2 X 3^6 + 2 X 3^7

3 + 2 ( 3^1 + 3^2 .... 3^7)

3 + 2 ( 3(3^7 - 1)/2) [using GP Sum formula]

3 + 3^8 - 3

3^8

Follow the Underlined & Bold Texts

3 + 3 + 3 + 2 X 3^2 + 2 X 3^3 + 2 X 3^4 + 2 X 3^5 + 2 X 3^6 + 2 X 3^7

3 + 2x3^1 + 2 X 3^2 + 2 X 3^3 + 2 X 3^4 + 2 X 3^5 + 2 X 3^6 + 2 X 3^7

3 + 2x3^1 + 2 X 3^2 + 2 X 3^3 + 2 X 3^4 + 2 X 3^5 + 2 X 3^6 + 2 X 3^7

3 + 2 ( 3^1 + 3^2 .... 3^7) [ GP => 3^1 + 3^2 .... +3^7]

Sum of GP = a(((r^n)-1)/(r-1))

Here, A = 3^1 , r = 3 , n = 7

So, 3 + 2 ( 3(3^7 - 1)/2)

3 + 3^8 - 3

3^8

Is it now better?

[email protected] wrote:3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 + 2 × 3^7 =

A. 3^7
B. 3^8
C. 3^14
D. 3^28
E. 3^30

Ans-B

Always look at the answer choices BEFORE you start to work.
Here, the answer choices are SPREAD OUT.
Thus, we can quickly BALLPARK the correct answer.
Since the greatest term = 2 * 3�, the sum of ALL the terms must be GREATER than 3�.
Eliminate A.

2 * 3� = 2 * 3³ * 3� = 2 * 27 * 81 ≈ 60*80 = 4800.
2 * 3� = (2 * 3�)/3 = 4800/3 = 1600.
2 * 3� = (2 * 3�)/3 ≈ 1600/3 ≈ 500.
The terms to the left of 2 * 3� will be relatively small and thus will have little effect on the final sum.
Thus, the correct answer ≈ 4800+1600+500 = 7000.

Look at the remaining answer choices.
C: 3¹� = 3� * 3� * 3� * 3² = 81*81*81*9 ≈ 80*80*80*10.
Thus, answer choices C, D and E are WAY too big.

The correct answer is B.

Dint get how u derive this step:

2* 3� = 2 * 3³ * 3� = 2 * 27 * 81 ≈ 60*80 = 4800.
2 * 3� = (2 * 3�)/3 = 4800/3 = 1600.
2 * 3� = (2 * 3�)/3 ≈ 1600/3 ≈ 500.
The terms to the left of 2 * 3� will be relatively small and thus will have little effect on the final sum.

GMATGuruNY wrote:

[email protected] wrote:3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 + 2 × 3^7 =

A. 3^7
B. 3^8
C. 3^14
D. 3^28
E. 3^30

Ans-B

Always look at the answer choices BEFORE you start to work.
Here, the answer choices are SPREAD OUT.
Thus, we can quickly BALLPARK the correct answer.
Since the greatest term = 2 * 3�, the sum of ALL the terms must be GREATER than 3�.
Eliminate A.

2 * 3� = 2 * 3³ * 3� = 2 * 27 * 81 ≈ 60*80 = 4800.
2 * 3� = (2 * 3�)/3 = 4800/3 = 1600.
2 * 3� = (2 * 3�)/3 ≈ 1600/3 ≈ 500.
The terms to the left of 2 * 3� will be relatively small and thus will have little effect on the final sum.
Thus, the correct answer ≈ 4800+1600+500 = 7000.

Look at the remaining answer choices.
C: 3¹� = 3� * 3� * 3� * 3² = 81*81*81*9 ≈ 80*80*80*10.
Thus, answer choices C, D and E are WAY too big.

The correct answer is B.