∆ 2 ⁄ ℎ = 4 3 4 = (∆ ℎ + ∆ ℎ + ∆ ℎ ) + 2 + 4 2 2 3 ℎ ℎ ∆ ℎ = ∆ ℎ = ∆ ℎ = = = ∆ ℎ ℎ ℎ ∆ ̅ ℎ ∆ ̅ ̅ − ̅ − ∆ ̅ ℎ ∆ ̅ ℎ ℎ ∆ = ̅ ∆ ̅ ℎ − ∆ ̅ ℎ ∆ ℎ = ̅ − ℎ ∆ ̅