设等差数列{an}的前n项和为Sn，且S5=a5．若a4≠0，则a7a4=．

Question

_nn554(a7)/(a4)=______．(a7)/(a4)a7a7a7a77a4a4a4a44

Answer 1

由已知S₅5=a₅
5
∴a₁1+a₂2+a₃3+a₄4=0
∴a₁1+a₄4=a₂2+a₃3=0，
∴a1＝−(3d)/(2)
∴(a7)/(a4)＝(−(3d)/(2)+6d)/(−(3d)/(2)+3d)＝3
故答案为3 a1＝−(3d)/(2)
∴(a7)/(a4)＝(−(3d)/(2)+6d)/(−(3d)/(2)+3d)＝3
故答案为3 1＝−(3d)/(2)3d3d3d222
∴(a7)/(a4)＝(−(3d)/(2)+6d)/(−(3d)/(2)+3d)＝3
故答案为3 (a7)/(a4)a7a7a77a4a4a44＝(−(3d)/(2)+6d)/(−(3d)/(2)+3d)−(3d)/(2)+6d−(3d)/(2)+6d−(3d)/(2)3d3d3d222+6d−(3d)/(2)+3d−(3d)/(2)+3d−(3d)/(2)3d3d3d222+3d＝3
故答案为3