প্রশ্নমতে,
\begin{aligned} & \lim_{x \rightarrow 0} \frac{1-\cos 3 x}{x^2} \\ & =\lim_{x \rightarrow 0} \frac{1-\cos 2 \cdot\left(\frac{3 x}{2}\right)}{x} \\ & =\lim_{x \rightarrow 0} \frac{2 \sin^2 \frac{3 x}{2}}{x^2} \quad\left[\begin{array}{c}x \rightarrow 0 \text{ হলে } \\ \frac{3 x}{2} \rightarrow 0<\text{ হবে }\end{aligned}\right] \\ & =\frac{\sin \frac{3 x}{2}}{\frac{3 x}{2}} \times \lim_{\frac{3 x}{2}} \frac{\sin \frac{3 x}{2}}{2} \times \frac{3 x}{2} \times \frac{3}{2} \times 2 \\ & =1 \times 1 \times \frac{3}{2} \times \frac{3}{2} \times 2 \\ & =\frac{9}{2} \mathrm{Ans} \\ & \end{aligned}
\end{array}\end{array}