几何分布Ge(p) 令q=1-p
E(x)=∑kp[q的(k-1)方]=p∑k[q的(k-1)方]=p∑(d[q的k方]/dq)=p*d∑[q(k)]/dq=p*d(1/(1-q))/dq=p/(1-q)^2=1/p