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Worked Examples · Example 4

Q.Evaluate lim⁡x→2x2+3x−4x3−1\displaystyle\lim_{x\to2}\frac{x^2+3x-4}{x^3-1} using the algebra of limits.

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Step 1 — Numerator's limit. x2+3x−4x^2+3x-4 is a polynomial, so by the algebra of limits (sum/product rules, reducing to direct substitution):

lim⁡x→2(x2+3x−4)=22+3(2)−4=4+6−4=6.\lim_{x\to2}(x^2+3x-4) = 2^2+3(2)-4 = 4+6-4=6.

Step 2 — Denominator's limit. Likewise, x3−1x^3-1 is a polynomial:

lim⁡x→2(x3−1)=23−1=8−1=7.\lim_{x\to2}(x^3-1) = 2^3-1 = 8-1=7.

Step 3 — Apply the quotient rule. Since the denominator's limit (77) is not zero, the quotient rule (§3) applies directly:

lim⁡x→2x2+3x−4x3−1=lim⁡x→2(x2+3x−4)lim⁡x→2(x3−1)=67.\lim_{x\to2}\frac{x^2+3x-4}{x^3-1} = \frac{\lim_{x\to2}(x^2+3x-4)}{\lim_{x\to2}(x^3-1)} = \frac{6}{7}. …

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