>>442
2項分布　E(X)=np , V(X)=npq
E(X)=Σ[k=0,n] kC[n,k](3/10)^k(7/10)^(n-k)
=Σ[k=1,n] nC[n-1,k-1](3/10)^k(7/10)^(n-k)
=(3/10)n Σ[k=1,n] C[n-1,k-1](3/10)^(k-1)(7/10)^{(n-1)-(k-1)}
=(3/10)n*1
=3

V(X)=Σ[k=0,n] k^2C[n,k](3/10)^k(7/10)^(n-k) - E(X)^2
=nΣ[k=1,n] kC[n-1,k-1](3/10)^k(7/10)^(n-k) -9
=3Σ[k=1,n] kC[n-1,k-1](3/10)^(k-1)(7/10)^(n-k) -9
=3Σ[k=1,n] (k-1)C[n-1,k-1](3/10)^(k-1)(7/10)^(n-k)
+3Σ[k=1,n] C[n-1,k-1](3/10)^(k-1)(7/10)^(n-k) -9
=3Σ[k=2,n] (n-1)C[n-2,k-2](3/10)^(k-1)(7/10)^(n-k) + 3*1-9
=3(n-1)(3/10)Σ[k=2,n] C[n-2,k-2](3/10)^(k-2)(7/10)^(n-k) -6
=(81/10)*1-6
=21/10