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Exercise 9.4 · Q8

Q.Evaluate the following limit:
[!FORMULA] lim⁡x→0tan⁡2xsin⁡5x\lim_{x\to0}\dfrac{\tan2x}{\sin5x}

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Step 1. Rewrite using the standard scalings.

tan⁡2xsin⁡5x=tan⁡2x2x⋅2xsin⁡5x5x⋅5x=25⋅tan⁡2x2xsin⁡5x5x.\frac{\tan2x}{\sin5x}=\frac{\dfrac{\tan2x}{2x}\cdot2x}{\dfrac{\sin5x}{5x}\cdot5x}=\frac{2}{5}\cdot\frac{\dfrac{\tan2x}{2x}}{\dfrac{\sin5x}{5x}}.

Step 2. Apply the standard limits. lim⁡θ→0tan⁡θ/θ=1\lim_{\theta\to0}\tan\theta/\theta=1 with θ=2x\theta=2x gives tan⁡2x/2x→1\tan2x/2x\to1; lim⁡θ→0sin⁡θ/θ=1\lim_{\theta\to0}\sin\theta/\theta=1 with θ=5x\theta=5x gives sin⁡5x/5x→1\sin5x/5x\to1. …

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