3分之1+15分之1+35分之1+......+399分之1
=1/(1*3)+1/(3*5)+1/(5*7)+......+1/(19*21)
=1/2 [1-1/3 +1/3 -1/5 +1/5- 1/7+......+1/19 -1/21]
=1/2 (1-1/21)
=1/2 *20/21
=10/21