Respuesta :

Answer:

a₁ = 10

Step-by-step explanation:

The common difference d is calculated as

d = a₃ - a₂ = - 20 - (- 5) = - 20 + 5 = - 15

Then

a₂ - a₁ = d , that is

- 5 - a₁ = - 15 ( add 5 to both sides )

- a₁ = - 10 ( multiply both sides by - 1 )

a₁ = 10