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Calculus / English

3 WAYS TO SOLVE LIMITS

Arnold Tutoring · YouTube · 5:00

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Three algebraic techniques resolve the lesson's indeterminate limits: factor and cancel, use a conjugate, and clear an inner denominator. The simplified functions agree near the target point even when the original expressions are undefined there.

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Chapters

0:45Factor and cancel near the point2:11Multiply by the conjugate4:30Clear the inner fraction

Learning script

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For x≠2x\ne2 near 2, x2+3x−10x2−2x=(x−2)(x+5)x(x−2)=x+5x\frac{x^2+3x-10}{x^2-2x}=\frac{(x-2)(x+5)}{x(x-2)}=\frac{x+5}{x}. Its limit as x→2x\to2 is 7/27/2. Cancellation concerns nearby inputs, not evaluation of the original expression at its missing point.

For lim⁡x→5x−5x+4−3\lim_{x\to5}\frac{x-5}{\sqrt{x+4}-3}, multiply by x+4+3x+4+3\frac{\sqrt{x+4}+3}{\sqrt{x+4}+3}. The difference of squares produces x−5x-5 in the denominator; after cancellation the remaining expression approaches 3+3=63+3=6.

Near x=−1x=-1, with x≠−1x\ne-1, multiplying the numerator and denominator of 3+3x1/x+1\frac{3+3x}{1/x+1} by xx gives 3x+3x21+x=3x\frac{3x+3x^2}{1+x}=3x. Thus the limit is −3-3. These examples start from 0/00/0 forms; a zero denominator alone is not a general license to cancel.

Knowledge cards

01

Factor and cancel near the point

For x≠2x\ne2 near 2, x2+3x−10x2−2x=(x−2)(x+5)x(x−2)=x+5x\frac{x^2+3x-10}{x^2-2x}=\frac{(x-2)(x+5)}{x(x-2)}=\frac{x+5}{x}. Its limit as x→2x\to2 is 7/27/2. Cancellation concerns nearby inputs, not evaluation of the original expression at its missing point.

02

Multiply by the conjugate

For lim⁡x→5x−5x+4−3\lim_{x\to5}\frac{x-5}{\sqrt{x+4}-3}, multiply by x+4+3x+4+3\frac{\sqrt{x+4}+3}{\sqrt{x+4}+3}. The difference of squares produces x−5x-5 in the denominator; after cancellation the remaining expression approaches 3+3=63+3=6.

03

Clear the inner fraction

Near x=−1x=-1, with x≠−1x\ne-1, multiplying the numerator and denominator of 3+3x1/x+1\frac{3+3x}{1/x+1} by xx gives 3x+3x21+x=3x\frac{3x+3x^2}{1+x}=3x. Thus the limit is −3-3. These examples start from 0/00/0 forms; a zero denominator alone is not a general license to cancel.

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  • Limits ExplanationAt 0:45
    Why this connection?

    For x≠2x\ne2 near 2, x2+3x−10x2−2x=(x−2)(x+5)x(x−2)=x+5x\frac{x^2+3x-10}{x^2-2x}=\frac{(x-2)(x+5)}{x(x-2)}=\frac{x+5}{x}. Its limit as x→2x\to2 is 7/27/2. Cancellation concerns nearby inputs, not evaluation of the original expression at its missing point. For lim⁡x→5x−5x+4−3\lim_{x\to5}\frac{x-5}{\sqrt{x+4}-3}, multiply by x+4+3x+4+3\frac{\sqrt{x+4}+3}{\sqrt{x+4}+3}. The difference of squares produces x−5x-5 in the denominator; after cancellation the remaining expression approaches 3+3=63+3=6. Near x=−1x=-1, with x≠−1x\ne-1, multiplying the numerator and denominator of 3+3x1/x+1\frac{3+3x}{1/x+1} by xx gives 3x+3x21+x=3x\frac{3x+3x^2}{1+x}=3x. Thus the limit is −3-3. These examples start from 0/00/0 forms; a zero denominator alone is not a general license to cancel.

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