Question

三分之一+十五分之一+三十五分之一+...+三百九十九分之一

Answer 1

=1/3+1/3*5+1/5*7+·····+1/19*21

=1/3+1/2(1/3-1/5)+1/2(1/5-1/7)+·····1/2(1/19-1/21)

=1/2(2/3+1/3-1/5+1/5-1/7+1/7·····+1/19-1/21)

=1/2(1-1/21)

=10/21