2²/1*3+4²/3*5+6²/5*7+8²/7*9+10²/9*11
=1+1/1*3+1+1/3*5+1+1/5*7+1+1/7*9+1+1/9*11
=5+(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)÷2
=5+(1-1/11)÷2
=5+5/11
=5又5/11
2²/1*3+4²/3*5+6²/5*7+8²/7*9+10²/9*11
=1+1/1*3+1+1/3*5+1+1/5*7+1+1/7*9+1+1/9*11
=5+(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)÷2
=5+(1-1/11)÷2
=5+5/11
=5又5/11