平方差公式：a²-b²=(a-b)(a+b)
(1-1/2²)(1-1/3²)(1-1/4²)...(1-1/98²)(1-1/99²)
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)...(1-1/98)(1+1/98)(1-1/99)(1+1/99)
=(1/2)×(3/2)×(2/3)×(4/3)×(3/4)×(5/4)×...×(97/98)×(99/98)×(98/99)×(100/99)
=(1/2)×1×1×...×1×(100/99)
=(1/2)×(100/99)
=50/99