1/1x3+1/3x5+1/5x7+1/7x9+1/9x11=?

=1/2×（1-1/3+1/3-1/5+1/5-1/7+17-1/9+1/9-1/11） =1/2×（1-1/11） =1/2×10/11 =5/11