only thing i can think of is rationalising it as an equation:
x=(n^2+n)*0.5
unfortunately it only works for a basic additive equation (1+2+3…), and doesn’t work for anything fancy like indices, though i’ve not tried it with multiplication or division. i based it on the idea that 1+4 and 2+3 both give you 5, so n=5, ∑(n)=15.
gotta love sigma.