Theorem: exercise3_8 (LeanCourse/Assignments/Assignment3.lean)

State before:
⊢ (fun x => x ^ 2) ⁻¹' {y | y ≥ 4} = {x | x ≤ -2} ∪ {x | x ≥ 2}
Tactic:
sorry
State after:
no goals