Question

（a-1）+(a^2-2)+.+(a^n-n)求和

Answer 1

S=（a-1）+(a^2-2)+.+(a^n-n)

=a+a^2+a^3+.+a^n-(1+2+3+.+n)

=a(1-a^n)/(1-a)-(1+an)*n/2

=(a-a^n+1)/(1-a)-(1+an)*n/2