Evaluate: 4 x 3x dx - Vidyadip

Question

Evaluate: $\int \cos \text{4 x} \cos 3\text{x dx}$

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Answer

$\text{I} = \int \cos \text{4 x} \cos 3\text{x dx} = \frac{1}{2} \int(\cos 7x + \cos x) \text{dx}$

$= \frac{1}{2} \bigg[ \frac{\sin 7x}{7} + \sin x\bigg] + c$or $\frac{1}{14} \sin 7 x + \frac{1}{2} \sin x + c$

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