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Worked Examples · Example 5

Q.Evaluate lim⁡x→0tan⁡7xsin⁡2x\displaystyle\lim_{x\to 0}\frac{\tan 7x}{\sin 2x}.

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Substitution gives 00\tfrac00. Reshape each piece to a standard form, matching arguments:

tan⁡7xsin⁡2x=tan⁡7x7x⏟→ 1⋅2xsin⁡2x⏟→ 1⋅7x2x.\frac{\tan 7x}{\sin 2x}=\underbrace{\frac{\tan 7x}{7x}}_{\to\,1}\cdot\underbrace{\frac{2x}{\sin 2x}}_{\to\,1}\cdot\frac{7x}{2x}.

As x→0x\to0: tan⁡7x7x→1\dfrac{\tan 7x}{7x}\to1 (since 7x→07x\to0), and 2xsin⁡2x→1\dfrac{2x}{\sin 2x}\to1 (reciprocal of sin⁡2x2x→1\tfrac{\sin2x}{2x}\to1). The leftover constant factor is 7x2x=72\dfrac{7x}{2x}=\dfrac72. …

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