Question

an÷ ; a1=5;r=4 a1+....+a2020=S2020=? progresie aritmetica​

Answer 1

Avem formula:

$S_n=n\cdot\dfrac{a_1+a_n}{2}$

Deci:

$S_{2020}=2020\cdot\dfrac{a_1+a_{2020}}{2}$

Formula termenului general:

$a_n=a_1+(n-1)\cdot r$

Deci:

$a_{2020}=5+(2020-1)\cdot 4=5+4\cdot2019=5+8076=8081$

$S_{2020}=2020\cdot\dfrac{5+8081}{2}=2020\cdot4043=8166860$