原式=30- 2×( 2/3+2/15+2/35+2/63+2/99+2/143+2/195+2/255 )
=30 -2×[(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) ]
=30 -2×( 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 )
=30 -2×( 1 -1/17 )
=30 -2×16/17
=30 -32/17
=30 -1又15/17
=28又2/17