a) 10+20+30+...+800=
=10(1+2+3+...+80)
=10(80*81:2)
=10*41*81
=410*81
=33210
b) 13+26+39+...+715=
=13(1+2+3+...+55)
=13(55*56:2)
=13*28*55
=20020
c) 21+42+36+...+1890=
=21(1+2+3+...+90)
=21(90*91:2)
=21*45*91
=85995
d) 101+202+303+...+909+1010+...+9898+9999=
=101(1+2+3+...+99)
=101(99*100:2)
=101*50*99
=499950
Sper ca te-am ajutat!
