- Topic
- L’Hôpital’s rule
- Difficulty
- Standard
- Suggested time
- 4 min
- Marks
- 3
L’Hôpital’s rule practice question 16
Evaluate , or decide that it does not exist.
Look at each piece near 0: , and . Which stay bounded, which tend to 0, and what does that suggest about the whole quotient?
The form is , since . Try the rule: the derivative of the top is . Does the new quotient have a limit?
When the quotient of derivatives has no limit, the rule says nothing. Split the original instead: , and use .
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
The form is 0/0, but the quotient of derivatives has no limit, so l’Hôpital’s rule is silent. Split the quotient into two factors whose limits are known.
Working
Product rule and chain rule: the derivative of sin(1/x) is cos(1/x) · (−1/x²), and the x² cancels.
Because cos(1/x) keeps swinging between −1 and 1, f′/g′ has no limit as x → 0, and the rule gives no information.
x / sin x → 1, and a bounded factor times x tends to 0.
The limit exists, although the rule could not find it.
Answer
The answer is zero.
Check
At x = 0.01 the quotient is about −0.0051, and in size it is at most x²/|sin x|, which tends to 0.
Where this goes wrong
Concluding that the limit does not exist because f′/g′ has none. The rule works in one direction only: if f′/g′ has a limit, f/g has the same one; if it has none, the rule is silent and another method is needed.
Leans on
Limits of products, from Limits
Try a similar question
Evaluate , or decide that it does not exist.Saved in this browser only.