Avem formula:
[tex]S_n=n\cdot\dfrac{a_1+a_n}{2}[/tex]
Deci:
[tex]S_{2020}=2020\cdot\dfrac{a_1+a_{2020}}{2}[/tex]
Formula termenului general:
[tex]a_n=a_1+(n-1)\cdot r[/tex]
Deci:
[tex]a_{2020}=5+(2020-1)\cdot 4=5+4\cdot2019=5+8076=8081[/tex]
[tex]S_{2020}=2020\cdot\dfrac{5+8081}{2}=2020\cdot4043=8166860[/tex]
