Question

一个等差数列前4项和为26,最后4项和为110,所有项之和为187,求 xiangshu

Answer 1

Sn=na1+n(n-1)d/2

=>

26 = 4a1 + 6d

187 = na1+n(n-1)d/2

67 = (n-4)a1 + (n-4)(n-5)d/2

联立应该求的出