a)A=1/1x2x3+1/2x3x4+...+1/98x99x100
=>2A=2/1.2.3+2/2.3.4+...+2/98.99.100
=>2A=2/1.2-2/2.3+2/2.3-2/3.4+....+2/98.99-2/99.100
=>2A=2/1.2 - 2/99.100
=>2A=1 - 1/50.99
=>2A=4949/4950
=>A=4949/9900
b)
B=(1-1/2)(1-1/2)......(1-1/n)
=>B=1/2.2/3.3/4.......(n-1)/n
=>B=1/n