lim (f(1-cos x)/(tan x)^2)
等价无穷小替换x^2~(tanx)^2
=lim (f(0 + 1-cosx)-f(0))/(1-cosx) * (1-cosx)/x^2
因为1-cosx->0,直接由导数的定义
=f'(0)*lim (1-cosx)/x^2
=1/2