- Topic
- L’Hôpital’s rule
- Difficulty
- Standard
- Suggested time
- 4 min
- Marks
- 2
L’Hôpital’s rule practice question 17
Evaluate the limit.
What do and each do as ? The rule needs a quotient, so which factor would you rather differentiate?
The form is . Rewrite it as , which has the form , keeping the logarithm on top.
One application leaves , another product. Rewrite it the same way and apply the rule again.
A single number.
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
Rewrite the product as a quotient with the logarithm on top, so that differentiating simplifies it. One application leaves a simpler product of the same kind, which one more rewrite settles.
Working
Dividing by 1/x is the same as multiplying by x.
Chain rule on (log x)²; multiplying the top and the bottom by x² clears the fractions.
Still 0 · ∞, so rewrite once more; the second application finishes it.
Answer
The answer is zero.
Check
At x = 0.001 the product is about 0.048, and at x = 0.000001 about 0.00019, falling toward 0.
Where this goes wrong
Answering +∞ because (log x)² grows without bound as x → 0⁺. The factor x shrinks to 0 at the same time; written as a quotient, the rule shows that x wins and the limit is 0.
Leans on
Behaviour of log x near 0, from Functions, graphs and inverses
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