Answer:
aₙ = 4(-1)ⁿ⁻¹(3)ⁿ⁻¹
Explanation:
The formula for the nth term of a geometric sequence is
aₙ = a₁rⁿ⁻¹
We can get the value for r by dividing aₙ by aₙ₋₁.
a₄/a₃ = -108/36
a₄/a₃ = -3
r = -3
aₙ = 4(-3)ⁿ⁻¹
aₙ = 4(-1)ⁿ⁻¹(3)ⁿ⁻¹
The (-1)ⁿ⁻¹ term makes even-numbered terms negative.