\(\text { Tìm giới hạn } B=\lim \limits_{x \rightarrow 0} \frac{\cos 2 x-\cos 3 x}{x(\sin 3 x-\sin 4 x)} \text { : }\)

* Hướng dẫn giải

\(B=\lim\limits _{x \rightarrow 0} \frac{2 \sin \frac{5 x}{2} \sin \frac{x}{2}}{-2 x \cos \frac{7 x}{2} \sin \frac{x}{2}}=-\lim\limits _{x \rightarrow 0}\left(\frac{5}{2} \cdot \frac{\sin \frac{5 x}{2}}{\frac{5 x}{2}}\right) \cdot \lim\limits _{x \rightarrow 0} \frac{1}{\cos \frac{7 x}{2}}=\frac{5}{2}\)