原式=lim(1-1/2)(1+1/2)(1-1/3)(1+1/3)*(1-1/4)*...*(1-1/N)*(1+1/N)
=lim1/2*3/2*2/3*4/3*3/4*...*(1-1/N)*(1+1/N)
=lim1/2*(1+1/N)
=1/2
原式=lim(1-1/2)(1+1/2)(1-1/3)(1+1/3)*(1-1/4)*...*(1-1/N)*(1+1/N)
=lim1/2*3/2*2/3*4/3*3/4*...*(1-1/N)*(1+1/N)
=lim1/2*(1+1/N)
=1/2