階差数列出せば良いんですよね。
途中式合ってます。
=3+ 3×(3の(n乗-1)-1)/2
3のn乗-1って(3のn乗)×(3の-1乗)って事、3の-1乗は1/3だから普通に分配法則で
=3+ 3のn乗-3/2
=6/2+ 3のn乗-3/2
=3のn乗+3/2
=1/2(3のn乗+3)
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