tính :
a) N = ( 1/2 - 1 ) . ( 1/3 - 1 ) . ( 1/4 - 1 ) .... ( 1/2016 - 1 ) . ( 1/2017 - 1 )
b) P = ( -1/1/2 ) . ( -1/1/3 ) . ( -1/1/4 ) ... ( -1/1/2015 ) . ( 1/1/2016 )
c) Q = 1/13 + 3/13.23 + 3/23.33 + ... + 3/2013.2023
d) R = 1/2017.2016 - 1/2016.2015 - 1/2015.2014 - ... - 1/( x+1999 ).( x+2000 )