Răspuns:
a)
S=1+2+3+...+80
S=80+79+78+...1
= 80 termeni
81+81+81+...+81
2×S=81×80
2×S=6480
S= 6480:2
S=3240
b)
S=2+4+6+...100
S=100+98+96+...2
=50 termeni
102+102+102+...+102
2×S=50×102
2×S=5100
S=5100:2
S=2550
c) ii imposibil 99:2 iti da cu virgula
sper ca te-am ajutat