3.3. For what least $a$ is the inequality $\frac{\sqrt[3]{\operatorname{ctg}^{2} x}-\sqrt[3]{\operatorname{tg}^{2} x}}{\sqrt[3]{\sin ^{2} x}-\sqrt[3]{\cos ^{2} x}}<a$ satisfied for all permissible $x \in\left(-\frac{3 \pi}{2} ;-\pi\right)$? Round the answer to the nearest hundredth if necessary.
Answer: $-2.52$ (exact value: $-2 \sqrt[3]{2}$ ).