Answer:
Step-by-step explanation:
common difference d=3-1=2
first term a=1
an=a+(n-1)d
2n-1=1+(l-1)2
2n-1=1+2l-2
2n-1=2l-1
l=n
(i used l for number of terms)
number of terms=n

Answer:
Wouldn't the third term be 21?
Answer:
I don't know sorry
it is clear that this is not online class
5^2+(-2)^2=29
magnitude=(29)^(1/2)=around 5.3852