It is known that it is the sum of the first n terms of Sn arithmetic sequence, and the equal proportion middle term of one third S3 and one fourth S4 is one fifth S5 And the mean term of one third S3 and one fourth S4 is 1
s3=3a1+3d
s4=4a1+6d
s5=5a1+10d
So (a1 + D) * (a1 + 1.5D) = (a1 + 2D) ^ 2
Reduction is 5D = - 3A1
a1+d+a1+1.5d=2
So we can synthesize the two forms
a1=4 d=-2.4
an=-2.4n+6.4
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