Respuesta :

Answer:

a + b = 15

Step-by-step explanation:

The common difference d between consecutive terms is

d = a₂ - a₁ = a - 18

d = a₄ - a₃ = - 3 - b

Thus

a - 18 = - 3 - b ( add b to both sides )

a + b - 18 = - 3 ( add 18 to both sides )

a + b = 15