tập xác địnhdomain

The domain of a function $f$ is the set of all values $x$ for which $f(x)$ can be computed. When a problem gives only a formula, the domain is the largest set of real numbers for which the formula makes sense. For example, $f(x)=\dfrac{1}{x-2}$ cannot be evaluated at $x=2$, so its domain is all real numbers except $2$.

The values most often excluded are those that make a denominator equal to $0$, those that make the expression under an even root negative, and those that make the argument of $\ln$ nonpositive. A real-world context can restrict the domain further: $A(r)=\pi r^2$ is the area of a circle with radius $r$, so $r>0$. For the composite function $\sqrt{\ln x}$, you need $x>0$ for $\ln x$ to be defined, and then $\ln x\ge 0$ for the square root to be defined, so $x\ge 1$.