1.

The sum of the products of 2n numbers `pm1,pm2,pm3, . . . . ,n` taking two at time isA. `-underset(r=1)overset(n)sumr`B. `underset(r=1)overset(n)sumr^(2)`C. `-underset(r=1)overset(n)sumr^(2)`D. none of these

Answer» Correct Answer - C
Let S be the required sum. We know that
`2underset(i)sumunderset(iltj" ")underset(j)sumx_(i)x_(j)=(underset(i=1)overset(n)sumx_(i))^(2)-underset(i=1)overset(n)sum x_(i)^(2)`
`:." "2S=(1-1+2-2+3-3+ . . . .+n-n)^(2)-2underset(r=1)overset(n)sumr^(2)`
`rArrS=-underset(r=1)overset(n)sumr^(2)`


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