( 10-x)！ =( 10-x)( 10-x- 1)( 10-x-2)( 10-x-3)…× 1

Keep the first three terms, and the rest are still expressed by factorial.

=( 10-x)(9-x)(8-x)(7-x)…× 1 =( 10-x)(9-x)(8-x)！