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The sum of products of 2n numbers ± 1, ± 2 , ± 3, ± 4... ± n taking two at a time is a) – ∑ r=1 n r ​b) ∑ r=1 n r^2 ​c) – ∑ r=1 n r^2 d)none of these Please explain the question and and its solution.. What does the question ask??

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2 years ago

```							we know that (a1+a2+a3+......+am)^2= sum of squares of all the terms + 2*(sum of product of these numbers taken two at a time)here, sum of ± 1, ± 2 , ± 3, ± 4... ± n =0so sum of product of these numbers taken two at a time= – (sum of squares of all the terms)/2but, sum of sqaures= (-1)^2+(1)^2+(-2)^2+(2)^2+.....+(-n)^2+n^2=2(1^2+2^2+.....+n^2)hence the final answer becomes – ∑r=1n r^2option C
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2 years ago
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