Attribute eq6-3 to eq6-2
from binomial coefficient formula
Attribute eq6-3 to eq6-2
$ \sum_i \binom{A + i}{i}\binom{B + K - i}{K - i}
$ = \sum_i \binom{A + i}{A}\binom{B + K - i}{B} ... eq4-3
$ = \sum_j \binom{j}{A}\binom{B + K - j + A}{B} ... A+i -> j
$ = \sum_j \binom{j}{A}\binom{N - j -1}{B} ... A+B+K+1 -> N
$ = \binom{N}{A + B + 1} ... eq6-2
$ = \binom{A+B+K+1}{A + B + 1} = \binom{A+B+K+1}{K}
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