Antiderivatives
An antiderivative of f is a function whose derivative is f. Finding one means reading the differentiation rules backwards, remembering that there is always a whole family of them, and using a given value to pick the one that fits. Velocity back to position is the standard application.
In this topic
- Taught
- Not in the course
- Questions
- 11
- Total marks
- 29
- Suggested time
- 47 min
- Difficulty
Introductory to Capstone
Show the 3 hardest
Key results
- The general antiderivative
For some constant , on an interval only; on a domain made of separate intervals, such as , each interval gets its own constant.
- Powers
For any constant , on an interval where is defined.
- The reciprocal
On or on , one interval at a time.
- Exponentials and trigonometric functions
For a constant , with in radians.
- Constant acceleration
For a constant acceleration , with and .
Read the theoryCLP-1 §4.1 Introduction to antiderivatives
Worked example
Find the function with and , and evaluate .
Strategy
Antidifferentiate each term, compensating for the chain-rule factor in cos(2x), fix the constant, and evaluate.
Working
Check: derivative is 4cos(2x) + eˣ.
Initial condition.
sin(π/2) = 1.
Answer
The answer is 2 plus e to the pi over 4, about 4.19.
Check
Differentiating 2 sin(2x) + eˣ gives back 4 cos(2x) + eˣ, and its value at x = 0 is 0 + 1 = 1, as required.
Where this goes wrong
Writing the antiderivative of 4cos(2x) as 4 sin(2x), which differentiates to 8cos(2x). The inner factor 2 must be divided out.
On the paper
Write the general antiderivative with its , find from the value given, then evaluate. Differentiate your answer to check it.
Leans on
The chain rule, from The chain rule and logarithms
Practice questions
5 of the 11, across every level. Open a solution to read the working, or answer the question yourself and it counts towards your progress.
- Introductory2 marks · 3 min
A function satisfies and . Find .
Answer this questionStrategy
Reverse the power rule on each term, remember the constant of integration, and fix the constant from the given value.
Working
Check by differentiating: 6x² − 4x + 1.
Initial condition.
Evaluate.
Answer
The answer is 3.
Check
Differentiating 2x³ − 2x² + x + 2 gives back 6x² − 4x + 1, and its value at x = 0 is 2, as required.
Where this goes wrong
Forgetting the constant and answering F(1) = 1. The derivative determines F only up to a constant; the condition F(0) = 2 is there to pin it down.
Leans on
The power rule, from Differentiation rules
Try a similar question
A function satisfies , and . Find . - Introductory1 mark · 2 min
Which of the following is the most general antiderivative of ?
Answer this questionStrategy
Read the differentiation rules backwards: cos differentiates to −sin, so −cos differentiates to sin; a constant is the derivative of a linear term. “Most general” means the + C is part of the answer.
Working
Each piece checks out.
Any two antiderivatives differ by a constant, so the family needs C.
Answer
The answer is negative 3 cosine x minus 2x plus a constant.
Check
Differentiating −3 cos x − 2x + C gives 3 sin x − 2 whatever C is, while 3 cos x − 2x + C gives −3 sin x − 2.
Where this goes wrong
Choosing 3cos x − 2x + C, with the sign of the cosine wrong. Differentiating that gives −3 sin x − 2, not 3 sin x − 2; always differentiate to check.
Leans on
The derivative of cos, from Exponential and trigonometric functions
Try a similar question
Find the function with and , and evaluate . - Standard3 marks · 5 min
A ball is thrown straight up from a height of 1 m with an initial velocity of 14.7 m/s. Its acceleration is a constant m/s². Find the maximum height it reaches.
Give the answer in metres.
Answer this questionStrategy
Constant acceleration integrates to linear velocity and quadratic position, with the two initial conditions fixing the two constants. The peak is where velocity is zero.
Working
Antiderivative of acceleration.
Antiderivative of velocity.
Peak height.
Answer
The answer is 12.025 metres.
Check
The speed falls steadily from 14.7 m/s to 0 in 1.5 s, averaging 7.35 m/s, so it climbs 7.35 × 1.5 = 11.025 m above its 1 m start.
Where this goes wrong
Dropping one of the constants of integration, typically the starting height of 1 m, which gives 11.025. Each antidifferentiation needs its own initial condition.
Leans on
Velocity and acceleration, from The definition of the derivative
- Challenging3 marks · 5 min
A function satisfies and . Find .
Answer this questionStrategy
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 . - Capstone5 marks · 8 min
A function is defined for all and satisfies for every , with and . Find .
Answer this questionStrategy
Two antiderivatives differ by a constant on an interval. The domain here is two intervals, so there are two independent constants, one per side, each fixed by its own condition.
Working
The general antiderivative of 1/x on each piece of the domain.
One condition per constant.
Evaluate on each side.
Sum.
Answer
The answer is 5.
Check
From x = 1 to e, and from −1 to −e, log|x| rises by 1, so f(−e) + f(e) = f(−1) + f(1) + 2 = 0 + 3 + 2 = 5.
Where this goes wrong
Writing f(x) = log|x| + C with a single C, which cannot satisfy both f(−1) = 0 and f(1) = 3. The “differ by a constant” theorem needs a single interval; across the gap at 0 the constants are independent.
Try a similar question
A function is defined for and satisfies there. Which of the following gives every such ?
Related topics
Needs
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