Consider the following mathematical expressions. 

a. [H⁺] = [HA]₀

b. [H⁺] = (K_a × [HA]₀)^1/2

c. [OH^-] = 2[B]₀

d. [OH^-] = (K_b × [B]₀)^1/2

For each expression, give three solutions where the mathematical expression would give a good approximation for the [H⁺] or [OH^-]. [HA]₀ and [B]₀ represent initial concentrations of an acid or a base.