- Topic
- L’Hôpital’s rule
- Difficulty
- Introductory
- Suggested time
- 3 min
- Marks
- 2
L’Hôpital’s rule practice question 5
Evaluate the limit.
Will differentiating and ever make this quotient simpler? As , which terms of the top and the bottom grow, and which fade away?
The form is . Apply the rule twice and compare the result with the limit you started from.
Instead, divide the top and the bottom by the fastest-growing term, , and use .
A fraction, or a decimal to two decimal places.
Theory UBC Math 100 §6CLP-1 §3.7Key results
Worked solution
Strategy
The rule goes round in circles here: two applications give back the quotient you started with. Leading terms settle it in one line: divide the top and the bottom by eˣ.
Working
Two applications return the original quotient, so the rule alone never finishes.
Divide every term by eˣ, the fastest-growing term; e−x/eˣ = e−2x.
e−2x → 0 as x → ∞.
Answer
The answer is three quarters.
Check
At x = 5 the quotient is already about 0.75003, and its e−x terms only shrink after that.
Where this goes wrong
Applying the rule again and again, or deciding that the limit does not exist because the rule never finishes. When the rule cycles, divide by the fastest-growing term instead.
Leans on
Limits at infinity, from Limits
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