1+2P+3P^2 +4P^3 +5P^4 +……..+KP^K-1
问题描述：
1+2P+3P^2 +4P^3 +5P^4 +……..+KP^K-1
答
S=1+2P+3P^2 +4P^3 +5P^4 +……..+KP^K-1①pS= P+2P^2+3P^3+4P^4+.+(k-1)P^K-1+KP^K②①-②：（1-P)S=1+P+P^2+P^3+...+P^K-1-KP^K=（1-p^K-1)/(1-P)-KP^KS=(1-p^K-1)/(1-P)^2-KP^K/(1-P)不化简了.打得好累....