- Topic
- L’Hôpital’s rule
- Difficulty
- Standard
- Suggested time
- 3 min
- Marks
- 2
L’Hôpital’s rule practice question 10
Evaluate .
As , what do and each do? To use l’Hôpital’s rule you need a quotient: which factor is easier to keep on top?
The form is , which is indeterminate but not a quotient. Rewrite as , which has the form .
Now apply l’Hôpital: .
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
A product of the form 0 · ∞ must be rewritten as a quotient before the rule applies. Put the logarithm on top; the power goes to the bottom as a reciprocal.
Working
Now a quotient of the allowed form.
One application, then simplify and substitute.
Answer
The answer is zero.
Check
At x = 0.01 the product is 0.0001 · log 0.01 ≈ −0.00046, tiny and negative, nowhere near −∞ or 1.
Where this goes wrong
Rewriting as x²/(1/log x) instead. That is also 0/0 in form but differentiating it makes things worse; put the logarithm in the numerator so its derivative simplifies.
Leans on
Behaviour of log x near 0, from Functions, graphs and inverses
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