harry3025
幼苗
共回答了19个问题采纳率：94.7% 举报
设公差为d
am=a1+(m-1)d=1/k (1)
ak=a1+(k-1)d=1/m (2)
(1)-(2)
(m-1)d-(k-1)d=1/k -1/m
(m-k)d=(m-k)/(km)
m≠k,m-k≠0
d=1/(km),代入(1)
a1+(m-1)/(km)=1/k
a1=1/(km)
S(mk)=kma1+km·(km-1)d/2
=km·[1/(km)]+km·(km-1)·[1/(km)]/2
=(km+1)/2
1年前
9
