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36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
 14 Jan 2012, 01:04
Question Stats:
57% (02:35) correct
42% (02:34) wrong based on 4 sessions
36^2 + 37^2 + 38^2 + 39^2 + 40^2 + 41^2 + 42^2+ 43^2 + 44^2 = (A)14400 (B)14440 (C)14460 (D)14500 (E)14520 Guys this is how I am trying to solve this. But after doing it for 15 minutes I gave up and think to post. No idea where I am getting this wrong and i don't have an OA either. Here we go: The question can be simplified as: 36^2 +{36+1}^2 +{36+2}^2 +{36+3}^2{36+4}^2{36+5}^2{36+6}^2{36+7}^2{36+8}^2 The above can be further simplified into 36^2 + {1296+1+72} + {1296+4+44} + ..................... + {1296+64+576} -------> [Using (a+b)^2 formula] -------------------------------------------(1) Now the (1) can be simplified into 1296 {1+74+149+226+.................+641} ==> 1296*2805 => This doesn't give me any of the above choices. Any idea guys where I am getting it wrong?
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enigma123 wrote: 36^2 + 37^2 + 38^2 + 39^2 + 40^2 + 41^2 + 42^2+ 43^2 + 44^2 =
(A)14400
(B)14440
(C)14460
(D)14500
(E)14520
Guys this is how I am trying to solve this. But after doing it for 15 minutes I gave up and think to post. No idea where I am getting this wrong and i don't have an OA either.
Here we go:
The question can be simplified as:
36^2 +{36+1}^2 +{36+2}^2 +{36+3}^2{36+4}^2{36+5}^2{36+6}^2{36+7}^2{36+8}^2
The above can be further simplified into
36^2 + {1296+1+72} + {1296+4+44} + ..................... + {1296+64+576} -------> [Using (a+b)^2 formula] -------------------------------------------(1)
Now the (1) can be simplified into
1296 {1+74+149+226+.................+641} ==> 1296*2805 => This doesn't give me any of the above choices. Any idea guys where I am getting it wrong? Ans is C Take the middle number here it is 40 now 36 = (40-4), 37 = (40-3) and so on on squaring this (40-4)^2 = 1600+16-320 and (40+4)^2 = 1600+16+320 on adding these two we get 2*1600+2*16 similarly we solve for all the above and finally we get => 8*1600+1600+2*(16+9+4+1) = 14460 (C)
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enigma123 wrote: 36^2 + 37^2 + 38^2 + 39^2 + 40^2 + 41^2 + 42^2+ 43^2 + 44^2 =
(A)14400
(B)14440
(C)14460
(D)14500
(E)14520 Approach #1:We have 9 terms, middle term is 40^2. Express all other terms as 40-x: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2==(40-4)^2+(40-3)^2+(40-2)^2+(40-1)^2+40^2+(40+1)^2+(40+1)^2+(40+3)^2+(40+4)^2. Now, when you expand these expressions applying (x-y)^2=x^2-2x+y^2 and (x+y)^2=x^2+2x+y^2 you'll see that -2xy and 2xy cancel out and we'll get: (40^2+4^2)+(40^2+3^2)+(40^2+2^2)+(40^2+1^2)+40^2+(40^2+1^2)+(40^2+2^2)+(40^2+3^2)+(40^2+4^2)==9*40^2+2*(4^2+3^2+2^2+1^2)=14,400+60=14,460Approach #2:The sum of the squares of the first n positive integers is given by: 1^2+2^2+...+n^2=\frac{n(n+1)(2n+1)}{6} (note that it's highly unlikely that you'll need it on the real GMAT test). For example the sum of the first 3 positive integers: 1^2+2^2+3^3=\frac{3(3+1)(2*3+1)}{6}=14. Now, we can calculate the sum of the squares of the first 44 positive integers and subtract from it the sum of the squares of the first 35 positive integers to get the answer: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2=\frac{44(44+1)(2*44+1)}{6}-\frac{35(35+1)(2*35+1)}{6}=14,460. Answer: C. Hope it helps.
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Bunuel wrote: enigma123 wrote: 36^2 + 37^2 + 38^2 + 39^2 + 40^2 + 41^2 + 42^2+ 43^2 + 44^2 =
(A)14400
(B)14440
(C)14460
(D)14500
(E)14520 Approach #1:We have 9 terms, middle term is 40^2. Express all other terms as 40-x: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2==(40-4)^2+(40-3)^2+(40-2)^2+(40-1)^2+40^2+(40+1)^2+(40+1)^2+(40+3)^2+(40+4)^2. Now, when you expand these expressions applying (x-y)^2=x^2-2x+y^2 and (x+y)^2=x^2+2x+y^2 you'll see that -2xy and 2xy cancel out and we'll get: (40^2+4^2)+(40^2+3^2)+(40^2+2^2)+(40^2+1^2)+40^2+(40^2+1^2)+(40^2+2^2)+(40^2+3^2)+(40^2+4^2)==9*40^2+2*(4^2+3^2+2^2+1^2)=14,400+60=14,460hm, before looking to ur solution, I solved the q. in this way- let a =36^2 then we have a+(a+1)+(a+2) (a+3) (a+4) (a+5) (a+6) (a+7) (a+8) I feel that it is just an arithmetic progression with mean=median so the sum of these numbers are 9*(a+4)=9*40^2=14400 later I saw ur solution - =9*40^2+2*(4^2+3^2+2^2+1^2)=14,400+60=14,460we seem to be in the same way, but using my method how to come to +2*(4^2+3^2+2^2+1^2)?
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LalaB wrote: Bunuel wrote: enigma123 wrote: 36^2 + 37^2 + 38^2 + 39^2 + 40^2 + 41^2 + 42^2+ 43^2 + 44^2 =
(A)14400
(B)14440
(C)14460
(D)14500
(E)14520 Approach #1:We have 9 terms, middle term is 40^2. Express all other terms as 40-x: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2==(40-4)^2+(40-3)^2+(40-2)^2+(40-1)^2+40^2+(40+1)^2+(40+1)^2+(40+3)^2+(40+4)^2. Now, when you expand these expressions applying (x-y)^2=x^2-2x+y^2 and (x+y)^2=x^2+2x+y^2 you'll see that -2xy and 2xy cancel out and we'll get: (40^2+4^2)+(40^2+3^2)+(40^2+2^2)+(40^2+1^2)+40^2+(40^2+1^2)+(40^2+2^2)+(40^2+3^2)+(40^2+4^2)==9*40^2+2*(4^2+3^2+2^2+1^2)=14,400+60=14,460hm, before looking to ur solution, I solved the q. in this way- let a =36^2 then we have a+(a+1)+(a+2) (a+3) (a+4) (a+5) (a+6) (a+7) (a+8) I feel that it is just an arithmetic progression with mean=median so the sum of these numbers are 9*(a+4)=9*40^2=14400 later I saw ur solution - =9*40^2+2*(4^2+3^2+2^2+1^2)=14,400+60=14,460we seem to be in the same way, but using my method how to come to +2*(4^2+3^2+2^2+1^2)? The problem is that 36^2, 37^2, 38^2, 39^2, 40^2, 41^2, 42^2, 43^2, and 44^2 DOES NOT form an AP. You assumed that a+1=36^2+1=37^2, a+2=36^2+2=38^2, ... but that's not correct: 37^2\neq{36^2+1}, 38^2\neq{36^2+2}, ... . Hope it's clear.
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Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
14 Jan 2012, 13:37
Bunuel, yep, u r right. my idea was wrong. thnx
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LalaB wrote:
I feel that it is just an arithmetic progression with mean=median
It is not an arithmetic progression, so if you assume that it is, you will get the wrong answer. An arithmetic progression is 'equally spaced'. If you just look at the smallest perfect squares, you can see that they are not equally spaced: 1, 4, 9, 16, 25... In fact the spacing gets larger the further you get into this list. You can use the spacing of perfect squares to answer this question. From the difference of squares, we have that 41^2 - 40^2 = (41 + 40)(41 - 40) = 81 So 41^2 = 40^2 + 81. Similarly, 42^2 = 41^2 + 83, and 39^2 = 40^2 - 79, and so on. Listing all of the values we need to sum: \begin{align*}
36^2 &= 40^2 - 79 - 77 - 75 - 73\\
37^2 &= 40^2 - 79 - 77 - 75 \\
38^2 &= 40^2 - 79 - 77 \\
39^2 &= 40^2 - 79 \\
40^2 &= 40^2 \\
41^2 &= 40^2 + 81 \\
42^2 &= 40^2 + 81 + 83 \\
43^2 &= 40^2 + 81 + 83 + 85 \\
44^2 &= 40^2 + 81 + 83 + 85 + 87
\end{align*}Now adding these in columns, we get (9)(40^2) + 4(81-79) + 3(83 - 77) + 2(85 - 75) + (87 - 73) = 9*1600 + 4*2 + 3*6 + 2*10 + 14 = 14,400 + 8 + 18 + 20 + 14 = 14,460 I'd still probably use the first method outlined in Bunuel's post above, but you can use the spacing of squares to get the answer if you look at the problem in the right way.
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Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
14 Jan 2012, 14:50
It would be easier if you think about this problem in this way:
(40-4)^2 + (40-3)^2 + (40-2)^2 + (40-1)^2 + (40-0)^2 + (40+1)^2+ (40+2)^2 + (40+3)^2 + (40+4)^2
all the 4*40*2 or 2* a*b terms will get cancelled out.
1600*9 + (4*4 + 3*3 + 2*2 + 1)*2 = answer = 14460
any other way would be too lengthy.
take middle term in such cases and try to formulate it in ways such as (a-1)^2 + (a+1)^2.
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IanStewart wrote: LalaB wrote:
I feel that it is just an arithmetic progression with mean=median
It is not an arithmetic progression, so if you assume that it is, you will get the wrong answer. An arithmetic progression is 'equally spaced'. If you just look at the smallest perfect squares, you can see that they are not equally spaced: 1, 4, 9, 16, 25... In fact the spacing gets larger the further you get into this list. You can use the spacing of perfect squares to answer this question. From the difference of squares, we have that 41^2 - 40^2 = (41 + 40)(41 - 40) = 81 So 41^2 = 40^2 + 81. Similarly, 42^2 = 41^2 + 83, and 39^2 = 40^2 - 79, and so on. Listing all of the values we need to sum: \begin{align*}
36^2 &= 40^2 - 79 - 77 - 75 - 73\\
37^2 &= 40^2 - 79 - 77 - 75 \\
38^2 &= 40^2 - 79 - 77 \\
39^2 &= 40^2 - 79 \\
40^2 &= 40^2 \\
41^2 &= 40^2 + 81 \\
42^2 &= 40^2 + 81 + 83 \\
43^2 &= 40^2 + 81 + 83 + 85 \\
44^2 &= 40^2 + 81 + 83 + 85 + 87
\end{align*}Now adding these in columns, we get (9)(40^2) + 4(81-79) + 3(83 - 77) + 2(85 - 75) + (87 - 73) = 9*1600 + 4*2 + 3*6 + 2*10 + 14 = 14,400 + 8 + 18 + 20 + 14 = 14,460 I'd still probably use the first method outlined in Bunuel's post above, but you can use the spacing of squares to get the answer if you look at the problem in the right way. I also used this approach, but it's really calculation intensive.
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Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
15 Jan 2012, 22:30
(A)14400
(B)14440
(C)14460
(D)14500
(E)14520
Sum of squares = (n)(n+1) (2n+1) /6
Sum of squares (44) - Sum of squares (35)
(44 * 45 * 89) /6 – (35 * 36 * 71)/6
(176220 – 89460) / 6 = 86760 / 6 = 14460
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Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
22 May 2013, 04:00
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Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2=
[#permalink]
22 May 2013, 04:00
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