Answer:
2520
Step-by-step explanation:
The n th term of an AP is
[tex]a_{n}[/tex] = a₁ + (n - 1)d
where a₁ is the first term and d the common difference.
To find the number of terms given [tex]a_{n}[/tex] = - 10 , d = - 2 and a₁ = 100, then
100 - 2(n - 1) = - 10 ( subtract 100 from both sides )
- 2(n - 1) = - 110 ( divide both sides by - 2 )
n - 1 = 55 ( add 1 to both sides )
n = 56 ← number of terms
Given n, a₁ and a₅₆ , then the sun of the terms is
[tex]S_{56}[/tex] = [tex]\frac{n}{2}[/tex] (a₁ + a₅₆ )
= [tex]\frac{56}{2}[/tex] (100 - 10) = 28 × 90 = 2520
