[數學]快算答案的算式

題目: 1999 x 19981998 – 1998 x 19991999

答案是 0

算式:

1999 x 19981998 – 1998 x 19991999
= 1999 x (19980000 + 1998) – 1998 x (1999000 + 1999)
= 1999 x 19980000 + 1999 x 1998 – 1998 x 1999000 – 1998 x 1999
= 1999 x 1998 x 10000 + 1999 x 1998 – 1998 x 1999 x 10000 – 1998 x 1999
= 0