1/1x5+1/5x9+1/9x13+1/13x17+1/17x21
=1/4x(1-1/5)+1/4x(1/5-1/9)+1/4x(1/9-1/13)+1/4x(1/13-1/17)+1/4x(1/17-1/21)
=1/4x(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)
=1/4x(1-1/21)
=5/21