解
原式
=1/2(1/11-1/13)+1/2(1/13-1/15)+1/2(1/15-1/17)+1/2(1/17-1/19)
=1/2[1/11+(1/13-1/13)+(1/15-1/15)+(1/17-1/17)-1/19]
=1/2(1/11-1/19)
=1/2×(8/209)
=4/209