>>685
E[(X-e^(ｰY))^2] = E[X^2] -2E[X]*E[e^(-y)] + E[e^(-2y)]
=V[X] + (E[X])^2 -2E[X]*E[e^(-y)] + E[e^(-2y)]
=np(1-p) + n^2p^2 - 2*np*(1/2) + 1/3
=np{(1-p)+np-1} + 1/3
=n(n-1)p^2 + 1/3