1) -1/[n(n+1)]=-1/n+1/(n+1)
2)
(―1*1/2)+(―1/2*1/3)+(―1/3*1/4+)+•••+(―1/2007*1/2008)+(-1/2008*1/2009)
=(-1+1/2)+(-1/2+1/3)+(-1/3+1/4)+...(-1/2007+1/2008)+(-1/2008+1/2009)
=-1+1/2009
=-2008/2009
1) -1/[n(n+1)]=-1/n+1/(n+1)
2)
(―1*1/2)+(―1/2*1/3)+(―1/3*1/4+)+•••+(―1/2007*1/2008)+(-1/2008*1/2009)
=(-1+1/2)+(-1/2+1/3)+(-1/3+1/4)+...(-1/2007+1/2008)+(-1/2008+1/2009)
=-1+1/2009
=-2008/2009