1/2-1/4+1/6-1/8+1/10-1/12+…+1/198-1/200 =(1-1/2+1/3-1/4+ ...+1/99-1/100)/2 =[(1+1/2+ ...+1/99+1/100)-2(1/2+1/4+ ...+1/98+1/100)]/2 =[(1+1/2+ ...+1/99+1/100)-(1+1/2+ ...+1/49+1/50)]/2 =(1/51+1/52+1/53...