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Finding Areas Between Lines & Curves (Cambridge O Level Additional Maths)
Revision Note
Area Between a Curve & a Line
How do I find the area between a curve and a line?
- The area enclosed will be the difference between …
- the area under the line and
- the area under the curve
- These can be found separately
- STEP 1
- Find the intersections of the line and the curve (if not given)
- STEP 2
- Find the area under a curve, RC, using definite integration
- STEP 3
- Find the area under a line, RL, either using definite integration or the area formulae for basic shapes
- STEP 4
- To find the area, R, between the curve and the line subtract the smaller area from the larger area
- If curve on top this will be RC – RL
- If line on top this will be RL - RC
- There may be easier ways to find the area under a line
Can I subtract before integrating?
- Yes - see the next note on "Area between two curves"
- Essentially you treat the line as a second curve
- It can be easier to make mistakes using this method
Examiner Tip
- Add information to any diagram provided
- Add axes intercepts, as well as intercepts between lines and curves
- Mark and shade the area you’re trying to find
- If no diagram is provided, sketch one!
Worked example
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Area Between Two Curves
What is the area between two curves?
- In general find the definite integral of “upper curve” – “lower curve”
- However this does depend on …
- … the area being found
- … if the curves intersect (and cross over)
- The area may have to be split into separate integrals
- The points at which curves intersect may need to be calculated
How do I find the area between two curves?
- STEP 1
- Find the intersections of the curves if needed
- STEP 2
- Form the integral …
- … using the intersections as limits
- … “upper curve” – “lower curve” …
- … and find the value of the integral
- Form the integral …
- STEP 3
- Repeat STEP 2 if more than one area needed
- STEP 4
- Add areas together
Examiner Tip
- If no diagram is provided sketch one, even if the curves are not accurate
- Add information to any given diagram as you work through a question
Worked example
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