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lim x → 0 sin 2 x ( cos 3 x − cos x ) x 3

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Question

\[\lim_{x \to 0} \frac{\sin 2x \left( \cos 3x - \cos x \right)}{x^3}\] 

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Solution

\[\lim_{x \to 0} \left[ \frac{\sin 2x \left( \cos 3x - \cos x \right)}{x^3} \right]\]
\[ = \lim_{x \to 0} \left[ \frac{\sin 2x \times - 2 \sin\left( \frac{3x + x}{2} \right) \sin\left( \frac{3x - x}{2} \right)}{x^3} \right]\]
\[ = - 2 \lim_{x \to 0} \left[ \frac{\sin 2x}{2x} \times \frac{\sin 2x}{2x} \times \frac{\sin x}{x} \right] \times 2 \times 2\]
\[ = - 8\]

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Chapter 29: Limits - Exercise 29.7 [Page 50]

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R.D. Sharma Mathematics [English] Class 11
Chapter 29 Limits
Exercise 29.7 | Q 37 | Page 50

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