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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.
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?
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.
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)
We have 9 terms, middle term is 40^2. Express all other terms as 40-x: \(36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2=\) \(=(40-4)^2+(40-3)^2+(40-2)^2+(40-1)^2+40^2+(40+1)^2+(40+2)^2+(40+3)^2+(40+4)^2\). 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:
Approach #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: \(36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2=\frac{44(44+1)(2*44+1)}{6}-\frac{35(35+1)(2*35+1)}{6}=14,460\).
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:
hm, 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=14,460 we 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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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:
hm, 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=14,460 we 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: \(37^2\neq{36^2+1}\), \(38^2\neq{36^2+2}\), ... .
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:
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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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:
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.
Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
08 Mar 2014, 19:28
Option C. Using the formula for calculating sum of squares of n natural nos.:[n(n+1)(2n+1)]/6 First calculate for n=44 then for n=35 And subtract second from first to get the answer.
Re: 36^2+37^2+38^2+39^2+40^2+41^2+42^2+43^2+44^2= [#permalink]
20 Jun 2015, 02:49
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