1.

If `(1+x)^(15)=C_0+C_1x+C_2x^2++C_(15)x^(15),`then find the su of `C_1+2C_3+3C_4++14 C_(15)dot`

Answer» Correct Answer - `13 xx 12^(14) + 1`
`C_(2)+2C_(3)+3C_(4)+"……."+14C_(15)`
`= underset(r=1)overset(14)sumr.^(15)C_(r+1)`
`= underset(r=1)overset(14)sum[(r+1)-1].^(15)C_(r+1)`
`= underset(r=1)overset(14)sum[(r+1)^(15)C_(r+1)-.^(15)C_(r+1)]`
`=underset(r=1)overset(14)sum(15.^(14)C_(r)-.^(15)C_(r+1))`
`= 15(2^(14)-1)-(2^(15)-.^(15)C_(0) - .^(15)C_(1))`
` = 13 xx 2^(14) + 1`


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