The domain of a function refers to the regions where the function is defined or has a value on a particular region.
4x2−19x2+1\; \frac{4x^{2} - 1}{\sqrt{9x^{2} + 1}}9x2+14x2−1 has a domain defined on all set of real numbers because the function is defined on the set of real numbers when the denominator 9x2+1≥0\sqrt{9x^{2} + 1} \ge 09x2+1≥0.
9x2+1≥0 ⟹ 9x2+1≥0\sqrt{9x^{2} + 1} \ge 0 \implies 9x^{2} + 1 \ge 09x2+1≥0⟹9x2+1≥0 which because of the square sign has a value for all values of x, be it negative or positive.