= 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