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Exercise 9.4 · Q9

Q.Evaluate the following limit:
[!FORMULA] lim⁡α→0sin⁡(αn)(sin⁡α)m\lim_{\alpha\to0}\dfrac{\sin(\alpha^n)}{(\sin\alpha)^m}

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Step 1. Rewrite numerator and denominator using the standard limit.

sin⁡(αn)(sin⁡α)m=sin⁡(αn)αn⋅αn(sin⁡αα)m⋅αm.\frac{\sin(\alpha^n)}{(\sin\alpha)^m}=\frac{\dfrac{\sin(\alpha^n)}{\alpha^n}\cdot\alpha^n}{\left(\dfrac{\sin\alpha}{\alpha}\right)^m\cdot\alpha^m}.

Step 2. Apply lim⁡θ→0sin⁡θ/θ=1\lim_{\theta\to0}\sin\theta/\theta=1. As α→0\alpha\to0, αn→0\alpha^n\to0 too (for n>0n>0), so sin⁡(αn)αn→1\dfrac{\sin(\alpha^n)}{\alpha^n}\to1; also sin⁡αα→1\dfrac{\sin\alpha}{\alpha}\to1, so (sin⁡αα)m→1\left(\dfrac{\sin\alpha}{\alpha}\right)^m\to1.

Step 3. Reduce to a power of α\alpha. Both bracketed factors →1\to1, so

lim⁡α→0sin⁡(αn)(sin⁡α)m=lim⁡α→0αn−m.\lim_{\alpha\to0}\frac{\sin(\alpha^n)}{(\sin\alpha)^m}=\lim_{\alpha\to0}\alpha^{n-m}. …

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