1.

The value of `sum_(r=0)^(40) r""^(40)C_(r)""^(30)C_(r)` isA. `40.^(69)C_(29)`B. `40.^(70)C_(30)`C. `.^(60)C_(29)`D. `.^(70)C_(30)`

Answer» Correct Answer - A
`underset(r=0)overset(40)sumr.^(40)C_(r).^(30)C_(r)=40underset(r=0)overset(40)sum.^(29)C_(r-1).^(30)C_(r)`
`= 40 underset(r=0)overset(40)sum.^(39)C_(r-1).^(30)C_(30-r)`
`= 40^(39+30)C_(r+1+30-r)`
`= 40.^(69)C_(29)`


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