Answer:
a₁ = 37
Step-by-step explanation:
In an arithmetic progression the common difference d is
d = a₂ - a₁ = a₃ - a₂ , that is
2x - 18 - (x + 1) = 2x - 1 - (2x - 18) ← distribute and simplify both sides
2x - 18 - x - 1 = 2x - 1 - 2x + 18
x - 19 = 17 ( add 19 to both sides )
x = 36
Thus
a₁ = x + 1 = 36 + 1 = 37
