`A= 1/1.2 + 1/3.4 + 1/5.6 +...+1/2015.2016`
`A= 1/1 - 1/2 + 1/3 - 1/4 + 1/5 -1/6+...+1/2015 - 1/2016`
`A= (1 + 1/3 + 1/5+...+1/2015) -(1/2 + 1/4 + 1/6+...+1/2016)`
`A= (1+1/2 + 1/3 + 1/4 +...+1/2015 + 1/2016) - 2(1/2 + 1/4 + 1/6 +...+1/2016)`
`A=1+1/2 + 1/3 + 1/4 +...+1/2015 + 1/2016 - 1 - 1/2 -1/3-...-1/1008`
`A= 1/1009 + 1/1010 +...+1/2016`
`=> B-A = 1/1008 + 1/1009 + 1/1010 +...+1/2016 - (1/1009 + 1/1010+ ...+1/2016)`
`=> B-A = 1/1008 + 1/1009 + 1/1010 + ...+ 1/2016 - 1/1009 - 1/1010 -...-1/2016`
`=> B-A= 1/1008`
Vậy `B-A= 1/1008`