Question

{an} ариф .прог аn=7+3n найти S20

Answer 1

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