应该不是lim(x→∞)而是lim(x→0)吧?
因为是周期函数,所以
-1
=lim(x→0)(f(1)-f(1-x))/2x
=lim(x→0)(f(5)-f(5-x))/2x
=f '(5)/2
f '(5)= -2