求和：(a-1)+(a²-2)+(a³-3)+…+(a^n-n)

求和：(a-1)+(a²-2)+(a³-3)+…+(a^n-n)
求和：(a-1)+(a²-2)+(a³-3)+…+(a^n-n)

求和：(a-1)+(a²-2)+(a³-3)+…+(a^n-n)
(a-1)+(a²-2)+(a³-3)+…+(a^n-n)
=(a+a²+a³+.+a^n)-(1+2+3+.+n)
=a(1-a^n)/(1-a) -(1+n)n/2
(前面是等比数列求和,后面是等差数列求和哦）