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The formula for the sum of squares of integer between 1 and n is n(

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Math Revolution GMAT Instructor
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The formula for the sum of squares of integer between 1 and n is n(  [#permalink]

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New post 25 Sep 2017, 00:52
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[GMAT math practice question]

The formula for the sum of squares of integer between 1 and n is n(n+1)(2n+1) / 6 = 1^2 + 2^2 + … + n^2. The is the value of 11^2 + 12^2 + … + 20^2?

A. 2470
B. 2475
C. 2480
D. 2485
E. 2490

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The formula for the sum of squares of integer between 1 and n is n(  [#permalink]

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New post 25 Sep 2017, 09:50
2
MathRevolution wrote:
[GMAT math practice question]

The formula for the sum of squares of integer between 1 and n is n(n+1)(2n+1) / 6 = 1^2 + 2^2 + … + n^2. The is the value of 11^2 + 12^2 + … + 20^2?

A. 2470
B. 2475
C. 2480
D. 2485
E. 2490


Sum of squares from 1 to 20 \(= \frac{20*21*41}{6} = 2870\) (use the formula given)

Sum of squares from 1 to 10 \(= \frac{10*11*21}{6} = 385\)

Hence sum of squares from 11 to 20 \(=\) Sum of squares from 1 to 20 \(-\) sum of squares from 1 to 10 \(= 2870-385 = 2485\)

Option D
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Re: The formula for the sum of squares of integer between 1 and n is n(  [#permalink]

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New post 27 Sep 2017, 01:39
=>

11^2 + 12^2 + … + 20^2
= ( 1^2 + 2^2 + … + 20^2 ) – (1^2 + 2^2 + … + 10^2)
= 20∙21∙41/6 - 10∙11∙21/6
= 2870 – 385
= 2485

Ans: D
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Re: The formula for the sum of squares of integer between 1 and n is n( &nbs [#permalink] 27 Sep 2017, 01:39
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