d=a2-a1=1-x+7=8-x
2x+1=a5=a2+3d=1-x+24-3x=25-4x
x=4
d=8-4=4
an=-7+(n-1)d=4n-11
Sn=2n^2-9n
设{an}是等差数列,a1=-7,a2=1-x ,a5=2x+1,sn是数列{an}的前n项和,求实数x 、求an和sn
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