bn=2/(n+1)=2*(1/n-1/(n+1))
所以：tn=2*(1-1/2+1/2-1/3+.+1/n-1/(n+1))=2*n/(n+1)?????????????{an}??,?????1/2,??a1+a3+a5+...+a99=60,??a1+a2+a4+...+a100??????????????a1+a3+a5+...+a99???????????????50????????d=0.5a1+a3+a5+...+a99=a1+(a1+2*d??+??a1+4*d??+...+(a1+98*d)=60///???????1+98/2=50???????50*a1+(2+4+6+...+98)*d=50*a1+2450*d=60???????a1a1+a2+a4+...+a100=a1+(a1+d)+(a1+3*d)+...+(a1+99*d)///????1+??99+1??/2=51=51*a1+(1+3+..+99)*d=51*a1+2500d??????????a1,???????a1+a3+a5+...+a99=60a2+a4+a6+...+a100=s??????50*d=s-60s=60+50*d?????a1s1=s+a1