分からない問題はここに書いてね464

>>48
Σ_{k = 0 ~ n} k(k - 1) nCk θ^k (1 - θ)^(n - k)
= Σ_{k = 2 ~ n} n!/((k - 2)!(n - k)!) θ^k (1 - θ)^(n - k)
= n(n - 1) θ^2 Σ_{j = 0 ~ n - 2} (n - 2)!/(j!(n - 2 - j)!) θ^j (1 - θ)^(n - 2 - j)
= n(n - 1) θ^2 Σ_{j = 0 ~ n - 2} (n - 2)Cj θ^j (1 - θ)^(n - 2 - j)
= n(n - 1) θ^2 (θ + (1 - θ))^ (n - 2) = n(n - 1) θ^2