- Topic
- L’Hôpital’s rule
- Difficulty
- Introductory
- Suggested time
- 3 min
- Marks
- 2
L’Hôpital’s rule practice question 4
Evaluate the limit.
As , what do and each do? Name the form before choosing how to compare them.
Top and bottom both grow without bound, so the form is . Differentiate each: and .
A single number.
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
An ∞/∞ race between a logarithm and a power. One application of the rule, then simplify the quotient of derivatives before taking the limit.
Working
Form ∞/∞, so the rule applies.
Divide the fractions, cancel √x, and the bottom still grows.
Answer
The answer is zero.
Check
At x = 10⁶ the quotient is about 0.014 and at x = 10¹² about 0.000028, heading to 0.
Where this goes wrong
Answering +∞ because log x grows without bound. The denominator grows too, and faster: any positive power of x eventually outgrows log x.
Leans on
Comparing logarithms and powers, from Power functions and first sketches
Try a similar question
Evaluate .Saved in this browser only.