Because the funeral is 1990 and this person is 100 years old,

So x >1990-100 =1890.

Namely 1890

According to x+n = n 2.

So n 2-n-x = 0.

X = N*(N- 1)

Between 1890 and 1990, it can be expressed as the product of two adjacent natural numbers, including:

43×44 = 1892

44×45 = 1980

That is, x = 1892 or 1980.

Because when x = 1980 and N=45, this is obviously not in line with the meaning of the question, so I omitted it.

Therefore, x = 1892, N = 44.

So this person was born in 1892.