a)
10+15+20+.....+2010-9-13-17-....-1609=
10+15+20+.......+2010=
ratia = 5
(2010-10):5+1=2000:5+1
=400+1
=401
(2010+10)*401:2=2020*401:2
=810020:2
=405010
9+13+17+......+1609=
ratia=4
(1609-9):4+1=1600:4+1
=400+1
=401
405010-324409=80601
b)
10+20+30+....+2020-9-18-27-.....-1818=
10+20+30+...+2020=
ratia=10
(2020-10):10+1=2010:10+1
=201+1
=202
(2020+10)*202:2=2030*202:2
=41006:2
=205030
9+18+27+...........+1818=
ratia=9
(1818-9):9+1=1809:9+1
=201+1
=202
(1818+9)*202:2=1827*202:2
=269054:2
=184527
205030-184527=20503
