what is it called if numbers $a$ and $b$ are such that their sum is equal to unity Question:
Is there a way to definitionally describe rational numbers $a$ and $b$ when $a+b=1$?
Answer:
My guess is that $a$ and $b$ may be defined as `unitary additive complements,' but this is just a guess.

In the same way that the error function, erf($x$), and the complementary error function, erfc($x$), sum to one (i.e., erf($x$) + erfc($x$) = 1) [1], so too then are $a$ and $b$ complementary under addition if $a + b = 1$.
[1]
https://en.wikipedia.org/wiki/Error_function#Complementary_error_function
