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Solution
\[= \lim_{x \to 0} \left[ \frac{- 2\sin\left( \frac{3x + 5x}{2} \right) \sin\left( \frac{3x - 5x}{2} \right)}{x^2} \right] \left[ \because cosC - cosD = - 2\sin\left( \frac{C + D}{2} \right)\sin\left( \frac{C - D}{2} \right) \right]\]
\[ = 2 \lim_{x \to 0} \left[ \frac{2\sin 4x \sin x}{x^2} \right] \left[ \because \sin\left( - \theta \right) = - \sin\theta \right]\]
\[ = 2 \lim_{x \to 0} \left[ \frac{\sin 4x}{4x} \times 4 \times \frac{\sin x}{x} \right]\]
\[ = 2 \times 4\]
\[ = 8 .\]
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