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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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