- Topic
- L’Hôpital’s rule
- Difficulty
- Standard
- Suggested time
- 4 min
- Marks
- 2
L’Hôpital’s rule practice question 26
A student evaluates the limit below as shown. Find the first line that is wrong.
Check each line against the statement of l’Hôpital’s rule: when may it be used, and what exactly does it replace with?
L’Hôpital’s rule replaces by . Is that the same as differentiating the fraction?
Differentiate and separately, then divide.
Theory UBC Math 100 §6CLP-1 §3.7Key results
Corrected working
Strategy
Check for 0/0, then replace the numerator and the denominator by their own derivatives.
Working
l’Hôpital: f′ over g′.
Substitute x = 0.
Answer
The answer is two.
Check
The quotient is the difference quotient of e2x at 0, so its limit is the slope there, 2; at x = 0.001 it is about 2.002.
Where this goes wrong
Differentiating the whole fraction with the quotient rule. L’Hôpital’s rule differentiates the top and the bottom separately.
Leans on
The derivative of e to the 2x, from The chain rule and logarithms
Try a similar question
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