Science:Math Exam Resources/Courses/MATH101/April 2011/Question 02 (a)

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MATH101 April 2011

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Question 02 (a)
Find the total area of the finite plane region lying between the curves $y=x$ and $y=x^{3}$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

Hint
Start by sketching the region.

Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

The curves $y=x$ and $y=x^{3}$ intersect at the points $(-1,-1),{\text{ }}(0,0),{\text{ }}(1,1).$ By symmetry, the area of the region bounded by the curves between $x=-1$ and $x=0$ is equal to the area of the region bounded by the curves between $x=0$ and $x=1$. So the total area will be equal to two times the area of the region between $x=0$ and $x=1$. Between $x=0$ and $x=1$, we have $x\geq x^{3}$; that is, the curve $y=x$ is above the curve $y=x^{3}$. So the area bounded by the curves and between $x=0$ and $x=1$ is $\int _{0}^{1}({\text{upper}}-{\text{lower}})dx=\int _{0}^{1}(x-x^{3})dx$ $=\left.\left({\frac {x^{2}}{2}}-{\frac {x^{4}}{4}}\right)\right\vert _{0}^{1}=\left({\frac {1^{2}}{2}}-{\frac {1^{4}}{4}}\right)-\left({\frac {0}{2}}-{\frac {0}{4}}\right)={\frac {1}{4}}.$ The total area of the region is two times this: $A=2\cdot {\frac {1}{4}}={\frac {1}{2}}.$

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