d s(t+△t) -st=im△t △s m At→0 △t △sn 0△ta0△tR R △Δϕ Δϕ s ˆ s ˆΔ Rt n R V R n t s t n t s t s t t s t dt ds t h t t t t t ™ ™ ™ ˆ ˆ ˆ ˆ = Δ Δ = Δ Δ = Δ Δ = Δ + Δ − = Δ → Δ → Δ → Δ → lim lim lim lim ( ) ( ) 0 0 0 0 ϕ