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Worked Examples · Example 5

Q.Evaluate lim⁡x→0sin⁡5xx\lim_{x \to 0} \dfrac{\sin 5x}{x}.

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The standard limit lim⁡x→0sin⁡xx=1\lim_{x \to 0}\dfrac{\sin x}{x}=1 applies only when the same expression appears both inside the sine and in the denominator — here it is 5x5x inside the sine but only xx in the denominator, so the expression must first be rewritten to match the standard form.

Multiply and divide by 55: sin⁡5xx=5×sin⁡5x5x\dfrac{\sin 5x}{x} = 5 \times \dfrac{\sin 5x}{5x}

As x→0x \to 0, the quantity 5x→05x \to 0 as well, so substituting u=5xu=5x: lim⁡x→0sin⁡5x5x=lim⁡u→0sin⁡uu=1\lim_{x \to 0} \dfrac{\sin 5x}{5x} = \lim_{u \to 0} \dfrac{\sin u}{u} = 1

Therefore: lim⁡x→0sin⁡5xx=5×1=5\lim_{x \to 0} \dfrac{\sin 5x}{x} = 5 \times 1 = 5 …

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