= 10000- 199- 199

= 10000- 199×2

= 10000-398

=9602

Note: 99×99 plus 99 becomes 99× 100, and 100 becomes 100× 100, which is more convenient to calculate [formula: (n+ 1)? =n？ +2n+ 1]. The original formula 2=999×778+333×666.

=999×778+333×(3×222)

=999×778+999×222

=999×(778+222)

=999× 1000

=999000