CSEC ADDITIONAL MATHEMATICS · SECTION 3 · LESSON C7

Definite Integrals, Curves from Gradients and Areas

Evaluating definite integrals, finding the equation of a curve from its gradient function and a point, and calculating areas bounded by a curve and lines in the first quadrant.

Introductory CalculusS3 B.7S3 B.8S3 B.9≈ 50 min🎮 3 games
Before you start

By the end of this lesson you can…

Step 1 · Watch

Explainer video

5 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.

LESSON C7 · EXPLAINER

Definite Integrals, Curves from Gradients and Areas

A definite integral is a number — and that number is an area. This is where integration earns its keep.

Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.

Explore in 3D

Area under a curve

Add rectangles and compare with the definite integral.

Step 2 · Read

Key notes

Copy these into your notebook in your own words — that is how they stick.

Definite integral

∫ₐᵇ f(x) dx = F(b) − F(a).

Reversal

∫ₐᵇ = −∫ᵇₐ; ∫ₐᵃ = 0.

Area

For f(x) ≥ 0: area under y = f(x) from a to b is ∫ₐᵇ f(x) dx.

Curve from gradient

y = ∫ (dy/dx) dx + C; use the given point to find C.

First quadrant

The syllabus areas lie in the first quadrant bounded by lines parallel to the y-axis.

Below the axis

The integral is negative; take the modulus for area.

Check

Estimate the area using a rectangle or triangle.

Units

Area in square units.

EXAM TIPS & COMMON MISTAKES

  • The constant C disappears in a definite integral — do not write it.
  • Substitute the UPPER limit first, then subtract the lower.
  • For a curve from its gradient, use the point to find C; do not leave C in the equation.
  • Sketch first: if the curve crosses the x-axis, split the area at the roots.
Play & practise

Games

3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness. More games in the Additional Mathematics Arcade.

Step 3 · Practise

Paper 1 practice — multiple choice

8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better. Work each item on paper first.

CSEC Paper 01 style · SO B.7

1. The value of ∫₁³ (2x + 1) dx is

Answer: B. [x² + x]₁³ = 12 − 2.
Adapted from CSEC P1 May/June 2024 (area of a region item)

2. The region R is enclosed by the x-axis, the curve y = x² + 2 and the lines x = 0 and x = 1. The area of R is

Answer: D. [x³/3 + 2x]₀¹.
CSEC Paper 01 style · SO B.8

3. The curve with gradient function dy/dx = 3x² − 2 passing through (1, 4) has equation

Answer: A. C = 5.
CSEC Paper 01 style · SO B.9

4. The area bounded by y = 4 − x² and the axes in the first quadrant is

Answer: A. 8 − 8/3.
CSEC Paper 01 style · SO B.7

5. The value of ∫₀^(π/2) cos x dx is

Answer: B. [sin x] = 1 − 0.
CSEC Paper 01 style · SO B.9

6. The area under y = x² from x = 1 to x = 3 is

Answer: C. 9 − 1/3.
CSEC Paper 01 style · SO B.7

7. If ∫₀^a (2x) dx = 9, the value of a (a > 0) is

Answer: A. [x²]₀^a = a² = 9.
CSEC Paper 01 style · SO B.7

8. Reversing the limits of a definite integral

Answer: D. ∫ₐᵇ = −∫ᵇₐ.
Step 4 · Apply

Paper 2 practice — structured questions

Write full answers, then reveal the mark scheme and tick the points you earned (25 marks in total). Show all your working — method marks count. Your answers are saved on this device.

CSEC Paper 02 style · Section 3 (Definite integrals and areas)

Question 1 [13 marks]

Answer all parts.

(a) Evaluate ∫₁³ (2x + 1) dx. [3]

MARK SCHEME — (a) (max 3)

(b) The gradient of a curve is dy/dx = 3x² − 2 and the curve passes through (1, 4). Find the equation of the curve. [4]

MARK SCHEME — (b) (max 4)

(c) Find the area of the region bounded by y = x² + 2, the x-axis and the lines x = 0 and x = 1. [3]

MARK SCHEME — (c) (max 3)

(d) Find the area of the region in the first quadrant enclosed by y = 4 − x² and the axes. [3]

MARK SCHEME — (d) (max 3)

CSEC Paper 02 style · Section 3 (Areas under curves)

Question 2 [12 marks]

The curve y = 6x − x² meets the x-axis at O and A.

(a) Find the coordinates of A. [2]

MARK SCHEME — (a) (max 2)

(b) Find the area of the region bounded by the curve and the x-axis. [4]

MARK SCHEME — (b) (max 4)

(c) Find the area under the curve between x = 1 and x = 3. [3]

MARK SCHEME — (c) (max 3)

(d) Evaluate ∫₀^(π/2) (sin x + cos x) dx. [3]

MARK SCHEME — (d) (max 3)

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Sources

Where this topic appears in CSEC papers

Find these in your own past-paper booklets for extra timed practice. Items above are written in the style of these papers; any adapted from a specific question say so.