原式=（1+2+3+4+5+6)+(1/3+2/15+2/35+2/63+2/99+2/143)
=21+[1/3+2/(3*5)+2/(5*7)+2/(7*9)+2/(9*11)+2/(11*13)]
=21+1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13
=21+2/3-1/13
=21又25/39