[code] a11+a1+a2(1+a1)(1+a2)+⋯+an(1+a1)(1+a2)⋯(1+an)=(1−11+a1)+(11+a1−1(1+a1)(1+a2))+⋯+(1(1+a1)(1+a2)⋯(1+an−1)−1(1+a1)(1+a2)⋯(1+an))=1−1(1+a1)(1+a2)⋯(1+an)≥1−1(21⋅a1)(21⋅a2)⋯(21⋅an)=1−12n=2n−12n
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