1/3*1/5+1/5*1/7＊＋1/7＊1/9＋⋯＋1/97＊1/99
=1/2*[（1/3-1/5)+(1/5-1/7)+...+(1/97-1/99)]
=1/2*(1/3-1/99)
=1/2*32/99
=16/99
1/3*1/5+1/5*1/7＊＋1/7＊1/9＋⋯＋1/97＊1/99
=1/2（1/3-1/5+1/5-1/7+1/7-1/9+....+1/97-1/99)
=1/2(1/3-1/99+
1/3x98/99
=98/297
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