lim<x→0>(1-cosx)/x = lim<x→0>(x^2/2)/x = 0,
则 x→0 时, 1- cosx 是 x 的高阶无穷小;
lim<x→0>(1-cosx)/x^2 = lim<x→0>(x^2/2)/x^2 = 1/2,
则 x→0 时, 1- cosx 是 x^2 的同阶无穷小;
lim<x→0>(1-cosx)/(x^2/2) = lim<x→0>(x^2/2)/(x^2/2) = 1,
则 x→0 时, 1- cosx 是 (x^2/2) 的等价无穷小;
lim<x→0>(1-cosx)/x^3 = lim<x→0>(x^2/2)/x^3 = lim<x→0>1/(2x) = ∞,
则 x→0 时, 1- cosx 是 x^3 的低阶无穷小。
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