lim(Δx→0) [ƒ(x₀ + 2Δx) - ƒ(x₀)]/Δx = 1/2lim(Δx→0) [ƒ(x₀ + 2Δx) - ƒ(x₀)]/(2Δx) · 2 = 1/2ƒ'(x₀) · 2 = 1/2ƒ'(x₀) = 1/4ƒ'(x) = lim(Δx→0) [ƒ(x + Δx) - ƒ(x)]/Δx <==导数定义
f'(x0)=lim2△x→0 [ f(x0+2△x)-f(x0)]/2△x=1/4
1/2