Question

{an} ариф .прог аn=7+3n найти S20

Answer 1

$a_{n}=7+3n\\a_{1}=7+3*1=7+3=10\\a_{20}=7+3*20=7+60=67\\S_{20}=frac{a_{1}+a_{20}}{2}*20=frac{10+67}{2}*20=77*10=770$