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1△χ就是從1開始,連續χ個正整數之和. 1,2,3,…,χ成等差數列,由等差數列求和公式得: χ(1+χ)/2=15 整理,得 χ^2+χ-30=0 (χ+6)(χ-5)=0 χ=5或χ=-6(舍去) χ=5
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