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2014-10-09T09:25:16+03:00
13+26+39+...+715=13+(13+13*1)+(13+13*2)+...+(13+13*54)=13*54+13*54*55/2=702+19305=20007
21+42+36+...1890=21+(21+21*1)=(21+21*2)+...+(21+21*89)=21*89+21*89*90/2=21*89*46=85974
101+202+303+...+909+1010+...+9898+9999
=101+(101+1*2)+(101+1*2)+...+(101+1*99)=101*99+101*99*100/2=101*99*50=499950
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