lim(x->2) cos(π/x)/(2-√(2x))
=lim(x->2) sin(π/2 - π/x)/(2-√(2x))
∵ x->2 , (π/2 - π/x) ->0 , 等价无穷小量替换：
sin(π/2 - π/x) ~ (π/2 - π/x)
=lim(x->2) (π/2 - π/x)/(2-√(2x))
分子分母同乘 [2+√(2x)]有理化
= π*lim(x->2) [(x-2)/2x] [2+√(2x)]/(4-2x)
= π*lim(x->2) (x-2)[2+√(2x)]/[2x*2(2-x)]
= -π*lim(x->2) [2+√(2x)]/[2x*2]
= -π/2