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Optimization: Rectangle Inscribed in Triangle

  1. Jun 2, 2008 #1
    [SOLVED] Optimization: Rectangle Inscribed in Triangle

    1. The problem statement, all variables and given/known data

    Please see this link. The problem I have is pretty much exactly the same as that dealt with in this excerpt.

    (focus on the bit with the heading "What is the biggest rectangle you can put inside a triangle")

    2. Relevant equations

    Shown in the link above.

    3. The attempt at a solution

    I basically want someone to please explain why we need not use a derivative. As you can see, the last sentence is chopped off and leaves me hanging. :smile:
  2. jcsd
  3. Jun 2, 2008 #2
    We see that x(a-x) is maximum if and only if after completing the square [tex](x-\frac{a}{2})^2 =0[/tex] for when x=? Therefore, the maximum rectangle has a height of what?
  4. Jun 2, 2008 #3
    Sorry, I didn't really answer your question. You don't need to use the derivative to find the maximum values of x and y because by completing the squares you can find the maximum value of x. Then, you can use x to find the area of that maximum triangle in terms of the area of the triangle.
  5. Jun 2, 2008 #4
    Could you still use a derivative though?
  6. Jun 2, 2008 #5
    "We need not use a derivative" does not imply that we can't use a derivative to solve for maximum x. So, yes you can use a derivative.
  7. Jun 2, 2008 #6
    Thank you very much for your help konthelion. I'm going to give it a shot using derivatives (I know nothing about completing the squares) and if you don't mind, I'd like you to have a peek at it as soon as I get round to posting it here...
  8. Jun 2, 2008 #7


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    It's very strange that a person would be able to use the derivative (a calculus topic) but not know how to complete the square (an algebra topic).
  9. Jun 3, 2008 #8
    I know the quadratic equation, but never learnt how it was derived (after some googling yesterday, I realised it's the result of completing the square)

    Were never taught it in school. Also, there's an eight year gap between the last time I did any maths and starting my BSc in Physics this year...

    It's tough going :smile:
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