1. A
2. B / A≡B
3. Av~B 1, Addition
4. Bv~A 2, Addition
5. ~AvB 4, Commutation
6. A⊃B 5, Material implication
7. ~BvA 3, Commutation
8. B⊃A 7, Material implication
9. (A⊃B)&(B⊃A) 6,8, Conjunction
10. A≡B 9, Material equivalence
[Note: Some books use -> for ⊃, <-> for ≡,
also •, ⋀, for & (and)]
Edwin