EX=[K*P*(1-p)^(K-1)]求和=[K*(1-P)^(K-1)]求和*p……P是常數，提到求和符號以外

= - [(1-P)^K]求導再求和*p

= - [(1-P)^K]求和再求導 * P 求和求導交換順序

= -[(1-P)/P] 再求導 * p 計算求和項

= -[1/P^2] *P 求導

= 1/p

同理算 E（X平方）

E(X^2)+EX= [ (K+1)*K*P*(1-P)^(K-1) ] 求和

= [ (K+1)*K*(1-P)^(K-1) ] 求和*p ……常數p提到求和項以外

= [ (1-P)^(K+1) ]求二階導 再求和*p

= [ (1-P)^(K+1) ]求和， 再求二階導，再*p

= [ (1-P)^2 / P] 求二階導，再*P

= [ 1/P-2+P] 求二階導，再 *P

= [ 2*P^(-3) ]*P

=2/P^2

方差=E（x平方）-（EX)的平方=[E(X^2)+EX ]-EX-(EX)^2

=2/P^2-1/P-(1/P)^2

=1/P^2-1/P=(1-P)/P