(1-1/2)+(1/2-1/3)+(1/3-1/4)+。。。。。(1/1999-1/2000)
=1+[-1/2+1/2]+[-1/3+1/3]+[-1/4+1/4]+...。。。。。[-1/1999+1/1999]-1/2000)
=1 - 1/2000
=1999/2000
(1-1/2)+(1/2-1/3)+(1/3-1/4)+。。。。。(1/1999-1/2000)
=1+[-1/2+1/2]+[-1/3+1/3]+[-1/4+1/4]+...。。。。。[-1/1999+1/1999]-1/2000)
=1 - 1/2000
=1999/2000