(1-1/2)(1-1/3)(1-1/4)...(1-1/1999)
=1/2 x 2/3 x 3/4 x...x1998/1999
=1/1999
所以原式=1999x1/1999=1
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(1-1/2)(1-1/3)(1-1/4)...(1-1/1999)
=1/2 x 2/3 x 3/4 x...x1998/1999
=1/1999
所以原式=1999x1/1999=1
1999*(1-1/2)(1-1/3)(1-1/4)(1-1/5)(1-1/6)......(1-1/1999)
= 1999*1/2*2/3*3/4*4/5...........*1998/1999
=1