Answer:
a₁ = -21 (Given)
a₂ = -28
a₃ = -35
a₄ = -42
Step-by-step explanation:
First term a₁ = -21 (Given)
…………………………………
Second term a₂ :
a₂ = 
…………………………………
Third term a₃ :
a₃ = 
…………………………………
Fourth term a₄ :
a₄ = 
Wouldn't it be answer b)?
which is similar to n:1 where 
<u>Step-by-step explanation:</u>
Here we have to Express in the form n:1 give n as a decimal 21:12 . Let's find out:
Given ratio as 21:12 . Let's convert it into n:1 , where n is decimal
⇒ 
⇒ 
⇒ 
⇒ 
⇒
{ dividing denominator & numerator by 4 }
⇒ 
⇒ 
⇒
which is similar to n:1 where 
Answer:sum =378 according to your new questions.
to find average temperature
use formula
average =sum of temperature /no of measuring day
=378/9=42 is your answer