- Topic
- Antiderivatives
- Difficulty
- Challenging
- Suggested time
- 5 min
- Marks
- 3
- Theory
- CLP-1 §4.1Key results
Antiderivatives practice question 8
A function satisfies and . Find .
Compare the factor in front with the exponent . How are they related, and which rule, run backwards, does that suggest?
Try : , so .
Now use to find . What is at ?
An exact expression, or a decimal to two decimal places.
Theory CLP-1 §4.1Key results
Worked solution
Strategy
When one factor is the derivative of the inside of the other, the whole expression is the derivative of a composition. Guess ex³, differentiate, and adjust the constant.
Working
Scale to match.
Initial condition.
Evaluate.
Answer
The answer is 2 times the quantity e minus 1, about 3.44.
Check
Differentiating 2ex³ gives 2 · 3x² · ex³ = 6x²ex³, and 2ex³ − 2 is 0 at x = 0, so F(1) = 2e − 2.
Where this goes wrong
Antidifferentiating the two factors separately, e.g. 2x³ · ex³. Products do not antidifferentiate factor by factor; the structure to spot is the chain rule.
Leans on
The chain rule, from The chain rule and logarithms
Try a similar question
Find the function with and , and evaluate .Saved in this browser only.