4.
1 A B
__________ = ______ + _____ multiply through by n ( n + 1)
n ( n + 1) n n + 1
1 = A( n + 1) + Bn
1 = (A + B)n + A equate terms
A + B = 0
A = 1
So B = -1
So
1 / [ n (n+1) ] = 1/ n - 1/ (n + 1)
So we have this sum
( 1/1 - 1/2 ) + ( 1/2 - 1/3) + ( 1/3 -1/4) + (1/4 - 1/5) + ....+ ( 1/ n - 1/ (n + 1) ) =
1 - 1/ ( n + 1) =
( n + 1) -1
_________ =
n + 1
n / ( n + 1)