等差数列An中,Sn为前N项的和S100=10,S10=100,问S110——?

问题描述:

等差数列An中,Sn为前N项的和S100=10,S10=100,问S110——?

解设每项相差m
S20=S10+(S10+10*10m)
S100=10S10+90m*10+80m*10+70m*10+60m*10+50m*10+40m*10+30m*10+20m*10+10m*10
S110=S100+S10+100m*10
=-110