3,2,-2,-11,-27…→an
階差を見ると、
-1,-4,-9,-16…→-n^2
n-1
an=3+Σ-k^2とわかる。
k=1
これを解くと、
an=3-1/6n(2n^2-3n+1)
=-1/6(2n^3-3n^2+n-18)
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