2²/1×3+4²/3×5+6²/5×7+8²/7×9+...+20²

题目有问题,按照规律,最后一个数应该是.+20²/19×21
2²/1×3+4²/3×5+6²/5×7+8²/7×9+...+20²/19×21
=4/1×3+16/3×5+36/5×7+64/7×9+...+400/19×21
=1+1/1×3+1+1/3×5+1+1/5×7+1+1/7×9+...+1+1/19×21
=1×10+1/2(1-1/3+1/3-1/5+1/5-.-1/21)
=10+1/2×20/21
=10又10/21