Gọi A=1/99x97-1/97x95-1/95x93-...-1/5x3-1/3x1
Suy ra A=-1/1x3-1/3x5-...-1/93x95-1/95x97-1/97x99
2A=-2/1x3-2/3x5-...-1/93x95-1/95x97-1/97x99
2A=-(2/1x3+2/3x5+...+1/93x95+2/95x97+1/97x99
2A=-(1/2-1/3+1/2-1/5+...+1/93-1/95+1/95-1/97+1/97-1/99)
2A=-(1/2-1/99)
2A=-97/198
A=-97/396