lim(ε→0) (C(K-ε,T)-C(K+ε,T))/2ε
= lim(ε→0) (C(K-ε,T)-C(K,T)+C(K,T)-C(K+ε,T))/2ε
= lim(ε→0) ((C(K-ε,T)-C(K,T))/2ε - (C(K+ε,T)-C(K,T))/2ε)
= lim(ε→0) (C(K-ε,T)-C(K,T))/(-ε) × -1/2 - lim(ε→0) (C(K+ε,T)-C(K,T))/ε × 1/2
= -1/2 × ∂C/∂K - 1/2 × ∂C/∂K
= -∂C/∂K

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