Răspuns:
Explicație pas cu pas:
a₈=a₁+7r=40
a₂₀=a₁+19r=-20
12r=-60
r=-5⇒a₁=75
a₇=a₁+6r=75-30=45
a₅₂=-125
r=-5
a₅₂=a₁+51r=-125
a₁-255=-125
a₁=130
a₂=3=a₁+r⇒[tex]a_{1}=3-\frac{2}{3}=\frac{7}{3}[/tex]
a₅=5=a₁+4r
3r=2
r=[tex]\frac{2}{3}[/tex]
a₂₈=a₁+27r=[tex]\frac{7}{3} +\frac{54}{3} =\frac{61}{3}[/tex]
[tex]S_{28}=\frac{(a_{1}+a_{28})\cdot28}{2}=\frac{(\frac{7}{3}+\frac{61}{3} ) }{2}=\frac{34}{3}[/tex]
