1*5/1+5*9/1+9*13/1+13*17/1+17*21/1

1*5/1+5*9/1+9*13/1+13*17/1+17*21/1=1/4(1-1/5)+1/4(1/5-1/9)+1/4(1/9-1/13)+1/4(1/13-1/17)+1/4(1/17-1/21)=1/4[1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21]=1/4(1-1/21)=1/4×20/21=5/21