1.

Let `f (n) =sum_(r=1)^n r^2 C_r^2`.Then, `f (5) =`A. `""^(8)C_(4)`B. `25xx""^(8)C_(4)`C. `25 xx""^(8)C_(5)`D. none of these

Answer» Correct Answer - b
`f(n) = sum_(r=1)^(n) r^(2) C_(r)""^(2)`
`rArr f(0)= sum_(r=1)^(n) (r ""^(n)C_(r))^(2) = n^(2) sum_(r=1)^(n) (""^(n-1)C_(r-1))^(2)`
`rArr f(n) = n^(2)""^(2n -2)C_(n-1)`
`rArr (5) = 25xx""^(8)C_(4)`


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