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There appear to be several expressions: -2x, 2-9x, 4x and 2-81 all of which are defined.
In Roman numerals 8.5 = VIIIS or SIX and 9.5 = VIIIIS or SXSo: VIIIS+VIIIIS = XVIII => 8.5+9.5 = 18 or octodecim in LatinAnd: SIX+SX = IIXX => (10-1.5)+(10-0.5) = (20-2) or duodevginti in LatinNote: 2*S = I, 5*I = V and 2*V = XValues: X = 10, V = 5, I = 1 and S = 1/2 or 0.5QED by David Gambell