原式
=50x(2/1x3+2/3x5+2/5x7+……+2/97x99)
=50x(1-1/3+1/3-1/5+1/5-1/7+……+1/97-1/99)
=50x(1-1/99)
=50x98/99
=4900/99 (99分之4900)