How do you square 19 in Roman numerals?

Answer 1

Due to rules made during the Middle Ages, today we write out 19 in Roman numerals as XIX.

But the Romans themselves would have probably wrote out 19 as XVIIII and then simplified it to IXX. In fact the Latin word for 19 is "undeviginti" which literally means one from twenty.

Hence:

IXX*IXX = -I*IXX + XX*IXX = +I-XX-XX+CCCC = CCCLXI (361)

In Hindu-Arabic numerals: -1(-1+20) + 20(-1+20) = +1-20-20+400 = 361