1x2+2x3+3x4+...+(nxn+1)
1/1x2 + 1/2x3 +1/3x4 +...+ 1/24x25
1/1x2 + 1/2x3 + 1/3x4 +...+ 1/2008x2009
1/1x2 + 1/2x3 + 1/3x4 + ... 1/99 x 100
tinh 1/1x2+1/2x3+1/3x4...1/2005x2006
A= 1/1x2 + 1/2x3 + 1/3x4 + .........+1/99x100
a = 1\1x2+1\2x3+1\3x4+........+1\49x50
A=1/1x2+1/2x3+1/3x4+.....+1/49x50
1/1x2+1/2x3+1/3x4+...+1/2013x2014+1/2014x2015