等差数列{an}的前n项之和为Sn。若S10=100,S100=10则S110=?解法:S10=a1+……+a10S100=a1+……+a100S100-S10=a11+……+a100=-9045(a11+a100)=-90a11+a100=-2S110=110(-2)/2=-110所以最后等于负110第四步以后就看不懂了,求教
等差数列