Respuesta :

Answer:

Step-by-step explanation:

an=a1+(n-1)d

d=21-20=1

a1=20,

an=29

29=20+(n-1)1

29-20=n-1

n=9+1=10

[tex]s_{n}=\frac{n}{2} (a_{1}+a_{n})=\frac{10}{2} (20+29)=5 \times 49=245[/tex]