1.

If `f(n)=sum_(r=1)^(n) r^(4)`, then the value of `sum_(r=1)^(n) r(n-r)^(3)` is equal toA. `(1)/(4){n^(2)(n+1)^(3)-4f(n)}`B. `(1)/(4){n^(3)(n+1)^(2)-4f(n)}`C. `(1)/(4){n^(2)(n+1)^(2)-4f(n)}`D. none of these

Answer» Correct Answer - B
We have, `underset(r=1)overset(n)sumr(n-r)^(3)=underset(r=1)overset(n)sum(n-r)r^(3)=underset(r=1)overset(n)sum(nr^(3)-r^(4))`
`=n underset(r=1)overset(n)sumr^(3)-underset(r=1)overset(n)sumr^(4)`
`=n{(n(n+1)^(2))/(2)}^(2)-f(n)`
`=(n^(3)(n+1)^(2))/(4)-f(n)=(1)/(4){(n^(3)(n+1)^(2)-4f(n)}`


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