a) (1/2-1)(1/3-1)(1/4-1)..............(1/1999-1)
=-1/2.(-2/3).(-3/4)....(-1998/1999)
=-1(-2)(-3)(-4).....(-1998)/-2(-3)(-4).....(-1999)
=1/1999
b)-1 1/2 * (-1 1/3) * (-1 1/4)............. (-1 1/1999)
= -3/2*-4/3*-5/4*...*-2000/1999
= -3*-4*-5*...*-2000/2*3*4*...*1999
= 2000/2
= 1000