- Topic
- L’Hôpital’s rule
- Difficulty
- Standard
- Suggested time
- 5 min
- Marks
- 3
L’Hôpital’s rule practice question 12
Evaluate the limit.
What does each fraction do as ? The rule works on a single quotient, so how could you make one from this difference?
The form is . Combine over a common denominator to get , which is .
Two applications are needed; simplify between them by multiplying through by .
A number, as a fraction or a decimal.
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
A difference of two infinities is rewritten as a single quotient, which then needs the rule twice. Clearing the 1/x factors between applications keeps the algebra short.
Working
Common denominator.
First application, then multiply numerator and denominator by x. Still 0/0.
Second application: derivative of x log x is log x + 1.
Answer
The answer is one half.
Check
At x = 1.001 the two terms are about 1000.4999 and 1000, a difference of about 0.4999, close to 1/2.
Where this goes wrong
Applying l’Hôpital to each fraction separately. The rule is for a single quotient; the two terms must be combined first.
Leans on
Combining fractions, from Functions, graphs and inverses
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