Write down the values of $\mathrm{????}$ which are not in the domain of the following functions:a$.\mathrm{????}(\mathrm{????})=\sqrt[\phantom{\mathrm{}}]{\mathrm{????}2-4\mathrm{????}}$b$.\mathrm{????}(\mathrm{????})=\frac{\mathrm{????}}{\mathrm{????}2-1}$