In equilibrium problems involving equilibrium constants that are small relative to the initial concentrations of reactants, we can often assume that the quantity x (which represents how far the reaction proceeds toward products) is small. When this assumption is made, we can ignore the quantity x when it is subtracted from a large number but not when it is multiplied by a large number. In other words, 2.5 - x ≈ 2.5, but 2.5x ≠ 2.5.

Explain why we can ignore a small x in the first case but not in the second.