- Topic
- L’Hôpital’s rule
- Difficulty
- Introductory
- Suggested time
- 3 min
- Marks
- 2
L’Hôpital’s rule practice question 8
Evaluate the limit.
As , which term of the top and which of the bottom grows fastest? Read carefully before you decide.
The form is . You can apply the rule twice, checking the form before each, or compare leading terms.
Divide the top and the bottom by , using and .
A whole number.
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
An ∞/∞ form that the rule settles in two applications and leading terms settle in one line. Either way, x² leads both the top and the bottom.
Working
By the rule: two applications, checking the form before each.
By leading terms: divide by x². log x / x² → 0 by one application of the rule, and e−x/x² → 0 outright.
Answer
The answer is three.
Check
At x = 10 the quotient is about 2.977, and at x = 100 about 2.9995, closing in on 3.
Where this goes wrong
Misreading e−x as a growing term, which makes the bottom look huge and the limit 0. As x → ∞, e−x → 0, so x² leads the top and the bottom and the limit is 3/1 = 3.
Leans on
Comparing powers, logarithms and exponentials, from Power functions and first sketches
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