Răspuns:
Explicație pas cu pas:
3.4
a1+7r+a1+2r-a1-5r=13 a1+4r=13 -a1-4r=-13
a1+4r+a1+2r=42 2a1+6r=42 a1+3r=21 -r=7 r=-7
a1=21-3r=21+21=42 a1=41; r=-7
3.3
S=80+83+...+161 an=80+(n-1)×3=161 (n-1)×3=81 n-1=27=n=28
S=(80+161)×28:2=241×14=3374
3.1
a1=8 a6=38 a6=a1+5r=38 8+5r=38 5r=30 r=6
an=8+6(n-1)=6n+2
S78=(8+6×78+2)×78/2=(10+468)×39=478×39=18642
3.2
a12=67 a7=82
a1+11r=67
a1+6r=82 5r=-15 r=-3 a1=67+33=100
an=100+99(-3)=100-297=-197
S203=[100+100+202(-3)]×203:2=(200-606)203/2=-406×203:2
=-203²=-41209
