999 *1999简便运算999 乘1999简便运算还有 824-396 763+298 9999*7778+3333*6666 1234567891的平方-1234567890*1234567892

999 * 1999
= (1000 - 1)(2000 - 1)
= 2000000 - 3000 + 1
= 1997000 + 1
= 1997001
824 - 396
= 824 - (400 - 4)
= 424 + 4
= 428
763 + 298
= 763 + 300 - 2
= 1063 - 2
= 1061
9999 * 7778 + 3333 * 6666
= 3333(3 * 7778 + 6666)
= 9999(7778 + 2222)
= 9999 * 10000
= 99990000
1234567891^2 - 1234567890 * 1234567892
= 1234567891^2 - (1234567891 - 1)(1234567891 + 1)
= 1234567891^2 - (1234567891^2 - 1^2)
= 1