y=2-x
lim(x->2) (2-x) tan(πx/4)
=lim(y->0) ytan[π(2-y)/4]
=lim(y->0) ytan(π/2-πy/4)
=lim(y->0) y.cot(πy/4)
=lim(y->0) y/tan(πy/4)
y->0 tan(πy/4) ~ πy/4 等价无穷小
=lim(y->0) y/(πy/4)
=4/π
