2*（1/3+1/15+1/35+1/63+1/99+1/143+1/195+1/255+1/323+1/399）=2/3+2/15+2/35+2/63+2/99+2/143+2/195+2/255+2/323+2/399=(1-1/3)+(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+(1/9-1/11)+(1/11-1/13)+(1/13-1/15)+(1/15-1/17)+(...