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2014-10-07T17:38:49+03:00
a)s=5+10+15+...+205=5(1+2+3+...+41)=5(41*42/2)=4305
b)s=1+5+9+....+405
=1+5+(5+1*4)+(5+2*4)+...+(5+100*4)=1+5+5+5+..+5+4*(1+2+....+100)=1+5*100+4*100*101:2=1+500+20200=20701