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1-2-3-4+5-6-7-8+9-10-11-12+…+37-38-39-40=(1-2-3-4)+(5-6-7-8)+(9-10-11-12)+…+(37-38-39-40)=-8-16-24-…-80=-88*10/2=-440先4個數一組,得出等差數列,再用首相加法算和,只是先不看-而已…
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