=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)……(1-1/99)(1+1/99)
=1/2 ×3/2×2/3 × 4/3×3/4 × ……×98/99 ×100/99
=1/2×100/99
=50/99