(1+3+5+...+97+99)-(2+4+...+98+100)
=(1+99)*50/2-(2+100)*50/2
=-50
18-（-13）+（-21）-（-42）
=18+13-21+42
=52
1/1x2+1/2x3+```````+1/2008x2009
=1-1/2+1/2-1/3+1/3-1/4+````````+1/2008-1/2009
=1-1/2009
=2008/2009
1-2+3-4+5-6+·····+99-100
(1+3+5+...+97+99)-(2+4+...+98+100)
=(1+99)*50/2-(2+100)*50/2
=-50
18-（-13）+（-21）-（-42）
=18+13-21+42
=52
1/1x2+1/2x3+```````+1/2008x2009
=1-1/2+1/2-1/3+1/3-1/4+````````+1/2008-1/2009
=1-1/2009
=2008/2009