生日的羅馬數字

1990.04.27 是 MCMXC. IV. XXVII

1990 = MCMXC
1000 = M
100 = C
10 = X
1990 = 1000 + (1000-100) + (100-10)
= M + CM (被減的數字放在前面, 被減的是100) + XC (被減的是10)
= M CM XC


4 = IV
5 = V
1 = I
4 = 5 - 1 = IV (被減的是1)


27 = XXVII
20 = 10 + 10 = XX
7 = 5 + 2 = 5 + (1 + 1) = V II
27 = 20 + 7 = XX + V + I + I = XXVII


所以 1990.04.27
是=> MCMXC. IV. XXVII


2012-05-07 11:35:36 補充：
又例如今天是 2012. 5. 7
羅馬數字是 MMXII. V. VII

2012 = 2000 + 10 + 2 = 1000 + 1000 + 10 + 1 + 1
= M + M + X + I + I
= MMXII

5 = V

7 = 5 + 1 + 1
= V + I + I = VII