Răspuns:
a) 10+20+30+....800= factor comun 10
=10(1+2+3+....80)=
=10(80x81):2= (am folosit suma lui gauss)
=10x6480:2=
=64800:2=32400
b)13+26+39+...715=
=13(1+2+3+...55)=
=13x(55x56):2=
=13x3080:2=
13x1540=20020
c)21+42+63+....1890=
21(1+2+3+...90)=
=21(90x91):2=
=21x8190:2=
=21x4095=85995
