pb:
1 − 1/2 + 1/3 − 1/4 + 1/5 − 1/6 + .... + 1/99 − 1/100 =
( 1 + 1/2 + 1/3 + 1/4 + 1/5 + .... + 1/99 + 1/100 ) − 2*( 1/2 + 1/4 + 1/6 + ... + 1/100 ) =
( 1 + 1/2 + 1/3 + 1/4 + ... + 1/50 ) + ( 1/51 + 1/52 + 1/53 + ... + 1/100 ) − ( 1 + 1/2 + 1/3 +
1/4 + ... + 1/50 ) = 1/51 + 1/52 + ... + 1/100