当x→0时,x的高阶无穷小量 1-cosx x^2 /2
Limit [ (1-cosx) / x ,x->0] = Limit [ 2 sin(x/2)^2 / x ,x->0]
= Limit [ 2 *(x/2)^2 / x ,x->0] = 0
当x→0时,x的同阶无穷小量 x+tan2x 3x
Limit [ (x + tan2x ) / x ,x->0] = Limit [ 3x / x,x->0] = 3
当x→0时,x的等价无穷小量 2/πcosπ/2(1-x) = （2/π）sin(πx /2)
Limit [ （2/π）sin(πx /2) / x ,x->0] = Limit [ x / x,x->0] = 1
没有 x 的低阶无穷小量.