Oh? I've never heard a mathematician state that 2 plus 2 might equal 4. Or a chemist say that a water molecule MIGHT contain two parts of hydrogen and one part oxygen. But maybe they're not real scientists.
1.9999999999 is deducted 2 and 1.9999999999 plus 1.9999999999 is 3.9999999998. 3.9999999998 is is deducted 4
H2O is water H3O is heavy water and exists in bodies of H2O. Both are water but heavy water is so named because of molecular weight difference.
H2O is water, and also heavy water. In ordinary water, the hydrogen atoms are almost entirely the light isotope of hydrogen, 1H, occasionally called protium. In heavy water, they are a heavier isotope of hydrogen, 2H, also called deuterium. The chemical formula of both, however, is the same.