Question

Find the first three terms of the arithmetic series described. n= 16 aₙ= 15 sₙ= -120.

Answer 1

Answer:

The first three terms are -30, -27 and -24

Step-by-step explanation:

The formula for nth term of a arithmetic series is given by:

aₙ = a₁ + (n - 1)d

Substitute n = 16 in the given equation:

a₁₆ = a₁ + (16 - 1)d

Where aₙ = a₁₆ = 15. Substitute in the given equation

15 = a₁ + 15d ⇒ Equation (i)

Sum of arithmetic sequence is given by:

Sₙ = n(a₁ + aₙ) / 2

Substitute n = 16 in the above equation:

S₁₆ = 16(a₁ + a₁₆) / 2

Where S₁₆= -120 and a₁₆=15, substitute:

-120 = 16(a₁ + 15)/2

-240 = 16(a₁ +15)

-15 = a₁ + 15

a₁ = -30

Substitute it in Equation (i)

15 = a₁ + 15d

15 = -30 + 15d

15d = 15+30

d = 45/15

d = 3

So

a₁ = -30

a₂ = a₁ + (2-1)d

a₂ = -30 + 3

a₂ = -27

a₃ = a₁ + (3-1)d

a₃ = a₁ + 2d

a₃ = -30 + 2(3)

a₃ = -30 +6

a₃ = -24