The reason for this is the following：

There are five odd numbers: 1, 3, 5, 7, 9; Even numbers are: 2,4,6,8.

The odd number is odd no matter how it is added or subtracted.

No matter how you add or subtract even numbers, the result is even numbers.

So the final result should be odd and even, or addition and subtraction between odd numbers.

And 10 is an even number, so it is impossible to get it!

To expand, you didn't know it in the fifth grade. There is an arithmetic called factorial. If a factorial symbol similar to an exclamation point is added after the formula, the formula is established.