等差数列{an}のa2-a1=8、a4がa2とa3の等比中項

等差数列{an}のa2-a1=8、a4がa2とa3の等比中項

d=a2-a1=8
a4^2=a2*a3
(a1+3d)^2=(a1+d)(a1+2d)
a1^2+6d*a1+9d^2=a1^2+3d*a1+2d^2
a1=-7d/3=-56/3
an=a1+(n-1)d
=-56/3+8(n-1)
=8n-80/3