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2014-10-14T16:54:13+03:00
6+11+16+21+......+506=(1+5)+(1+5*2)+(1+5*3)+(1+5*4)+.........+(1+5*101)=1*101+5+5*2+5*3+5*4+..........+5*101=101+5(1+2+3+........+101)=101+5*101*102:2=101+25755=25856

4+9+14+19+......+1004=4+(4+5)+(4+5*2)+(4+5*3)+............+(4+5*200)=4*201+(5+5*2+5*3+....5*200)=4*201+5*(1+2+3+...+200)=4*201+5*200*201:2=804+100500=101304