直接等价无穷小
F(x) ->0 时候
因为 (1+F(x) )^k - 1 ~ kF(x)
所以a(x) = [ (1+3x^3)^(1/3) - 1 ] - [ (1- 3x^3)^(1/3) - 1] (减一加一)
因为 x -> 0 时候
a(x) /(Ax^k) =1
所以
[ (1+3x^3)^(1/3) - 1 ] /(Ax^k) - [ (1- 3x^3)^(1/3) - 1] /(Ax^k) =1
等价无穷小替换
2x^3 / (Ax^k) =1
A=2
k=3
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