1/1*2 +1/2*3 +1/3*4......+1/19
1/1*2+1/2*3+1/3*4...1/19*20=?
(1-1/2-1/3-...-1/2009)(1/2+1/3+1/4...+1/2010)-(1-1/2-1/3-...
1+2+3+4.+10000
-1-2-3-4...-100
1+2+3+4.+1000
1+1+2+3+4.+99
1/2*1/3+1/3*1/4+1/4*1/5.+1/18*1/19+1/19*1/20
1/1+2+1/1+2+3+1/1+2+3+4+...+1/1+2+3+4...+99
1/1*2+1/2*3+1/3*4.+1/2011*2012+1/2012*2013
1/1*2+1/2*3+1/3*4...+1/99*100+1/(2011*2012)
1+2+3+4.+n(n+1)×1/2
观察下列各式:1/1*2=1/1-1/2,1/2*3=1/2-1/3,1/3*4=1/3-1/4.