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2014-09-25T19:44:29+03:00
3(1+2+3+....+1000)+(3+4+5+.....+1000)×(-3)=
=
3(1+2+3+....+1000)-(3+4+5+.....+1000)×3=
=3(
1+2+3+....+1000-3-4-5-....-1000)=
=3(1+2)=
=9