Tn=S1+S2+S3+...+Sn=na1+(n-1)a2+.+an=a1[n+(n-1)q+(n-2)q^2+.+q^(n-1)]
qTn=a1[nq+(n-1)q^2+(n-2)q^3+.+q^(n)]
错位相减法(q-1)Tn=a1[q+q^2+q^3+.+q^(n-1)+q^n - n]=a1q[(1-q^n)/(1-q)]-na1={[a1q-a1q^(n+1])/(1-q)}-[na1a1],Tn=【a1q[(1-q^n)/(1-q)]-na1】/[q-1]
考试前才复习过数列,这么快就用上了