2²/1×3+4²/3×5+6²/5×7+…+20²/19×21=1+1/1x3+1+1/3x5+1+1/5x7+...+1+1/19x21=1x19+1/2(1-1/3)+1/2(1/3-1/5)+1/2(1/5-1/7)+...+1/2(1/19-1/21)=19+1/2(1-1/21)=19+10/21=19又10/21