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( 2ⁿ + 2ⁿ ⁺¹ + 2ⁿ ⁺² + 2ⁿ ⁺³) ⋮ 15
2ⁿ + 2ⁿ ⁺¹ + 2ⁿ ⁺² + 2ⁿ ⁺³= 2ⁿ + 2·2ⁿ + 2²·2ⁿ + 2³·2ⁿ = 2ⁿ(1+2+4+8)=
2ⁿ · 15⋮ 15