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Worked Examples · Example 4

Q.Evaluate lim⁡x→0sin⁡5x3x\displaystyle\lim_{x\to 0}\frac{\sin 5x}{3x}.

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Substitution gives sin⁡00=00\tfrac{\sin0}{0}=\tfrac00. Use lim⁡u→0sin⁡uu=1\lim_{u\to0}\dfrac{\sin u}{u}=1, but the argument 5x5x must match the denominator. Rewrite:

sin⁡5x3x=sin⁡5x5x⋅5x3x=53⋅sin⁡5x5x.\frac{\sin 5x}{3x}=\frac{\sin 5x}{5x}\cdot\frac{5x}{3x}=\frac{5}{3}\cdot\frac{\sin 5x}{5x}.

As x→0x\to0, also 5x→05x\to0, so sin⁡5x5x→1\dfrac{\sin 5x}{5x}\to1. Hence the limit is 53⋅1=53\dfrac53\cdot1=\dfrac53. …

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