1/1*3+1/3*5+1/5*7+。。。。。。。+1/49*51=(1-1/3)*1/2+(1/3-1/5)*1/2+(1/5-1/7)*1/2+...+(1/49-1/51)*1/2=(1-1/51)*1/2=50/51*1/2=25/51
