1/1*3+1/3*5+1/5*7+1/7*9.1/17*19+1/19*21=1/2*(1-1/3)+1/2*(1/3-1/5)+1/2*(1/5-1/7)+1/2*(1/7-1/9)+.+1/2*(1/17-1/19)+1/2*(1/19-1/21)=1/2*(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+.+1/17-1/19+1/19-1/21)=1/2*(1-1/21)=1...