a) 10+15+20+ …........ +2010
-9 -13-17...................1609
----------------------------------------
1+ 2+ 3 ..................... 401 = 401*402/2=80601
b)10+20+30+ … +2020
-9-18- 27-…...-1818
_______________________
1+2 3+............... 202=202*203/2= 20503
d) 400 000+40 000+4 000+ 400+40+4 - 3-30-300-3 000-30 000-300 000=
= 4 +40 +400+4000+ 40000+ 40000-
- 3 - 30 -300 - 3000- 30000 - 30000
---------------------------------------------------
1+ 10 +100 +1000+10000+ 100000= 111 111