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ACARA ACMMM125, Definite integral as area under a curve

Year 11-12 · Mathematical Methods

Interpret the definite integral of f(x) from a to b as the area under the curve y=f(x) when f(x) is positive over the interval.

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Teaching guide

What students learn

Interpret the definite integral of f(x) from a to b as the area under the curve y=f(x) when f(x) is positive over the interval.

43
Area = 4 × 3 = 12 square units
How to teach it

For a curve that stays above the x-axis, the area between it and the axis from a to b is exactly the definite integral. Sketch the region first, then evaluate F(b) - F(a). Give the area in square units.

Worked example
Find the exact area of the shaded region between the curve y = 3x^2 + 5x + 5 and the x-axis, from x = 0 to x = 3. (The curve stays above the x-axis on this interval.)
Answer: 129/2 square units
Why: Area = definite integral of (3x^2 + 5x + 5) dx from 0 to 3 = F(3) - F(0) where F(x) = x^3 + (5/2)x^2 + 5x; = 129/2 - (0) = 129/2.

Generated and checked in code, so it is correct.

Watch out for

Assuming the definite integral always equals the area, it only does so where the curve is above the axis; below the axis the integral is negative (a signed area), which is why this introduction keeps the curve positive.

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More Year 11-12 Mathematical Methods codes: ACMMM114 · ACMMM116 · ACMMM117 · ACMMM121 · ACMMM122 · ACMMM131 · ACMMM132
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Australian Curriculum content descriptions (c) ACARA, used under CC BY 4.0. Descriptions are paraphrased. See australiancurriculum.edu.au.