110+220+330+440+550= 110 + 110*2 + 110*3 + 110*4 + 110*5 = 110*(1+2+3+4+5) = 110 * 15 = 1650
5+10+15+20+25+30+35= 5* (1 +2+3+4+5+6+7) = 5*28 = 140
1999*2000-1998*1999-2*1998 =
= 1999*(2000-1998) -2*1998 =
= 1999*2 -2*1998 =
=2 * (1999-1998)=2
11001+22002+33003+44004+55005= 11001 * (1+2+3+4+5) = 11001 *15=165015
