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