PMF 증명: $X$또는 $Y$의 값을 안다는 가정하에 식을 전개한다.
$$ \begin{aligned} P(X + Y = k) &= \sum_{j=0}^{k}P(X + Y = k | X = j)P(X = j) \\ &= \sum_{j=0}^{k}P(Y = k - j|X=j)\binom{n}{j}p^jq^{n-j} \\ &= \sum_{j=0}^k\binom{m}{k-j}p^{k-j}q^{m-k+j} \binom{n}{j}p^jq^{n-j} \\ &= p^kq^{m+n-k}\sum_{j=0}^{k}\binom{m}{k=j}\binom{n}{j} \\ &= p^kq^{m+n-k}\binom{m+n}{k} \end{aligned} $$
$$ \sum_{k=0}^w P(X = k) = \large \frac{1}{b+w \choose n}\sum_{k=0} ^w {w \choose k}{b \choose n-k} = {b+w \choose n} \large {\frac {1} {b+w \choose n}} $$