9乘以11分之2加11乘以13分之2加13乘以15分之2加15乘以17分之2加17乘以19分之2
=1/9-1/11+1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19
=1/9-1/19
=10/171