=9999*7777+3333*3*2222
=9999*7777+9999*2222
=9999*(7777+2222)
=(10000-1)*9999
=10000*9999-1*9999
=99990000-9999
=99980001
9999*7777+3333*6666简便运算
=9999*7777+3333*3*2222
=9999*7777+9999*2222
=9999*(7777+2222)
=(10000-1)*9999
=10000*9999-1*9999
=99990000-9999
=99980001