1/11*13+1/13*15+1/15*17+1/17*19+1/19*21
=(1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19+1/19-1/21)÷2
=(1/11-1/21)÷2
=10/231÷2
=5/231
1/11*13+1/13*15+1/15*17+1/17*19+1/19*21
=(1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19+1/19-1/21)÷2
=(1/11-1/21)÷2
=10/231÷2
=5/231