1/3*1+1/3*5+1/5*7+A+1/17*19+1/19*21
=1/2[1-1/3]+1/2[1/3-1/5]+...+1/2[1/19-1/21]
=1/2[1-1/3+1/3-1/5+...+1/19-1/21]
=1/2[1-1/21]
=10/21