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Homework Help: Maximum and minimun problem

  1. May 21, 2006 #1
    1. Find the dimensions of the rectangle with largest area which can be cut from a circle with equation x^2+ y^2= 4

    this is the question but i got stuck half way when i was differentiating the equation

    how do i work this out :
    [square root of (4-y^2)] + ([-y ^2] \ square root[ 4- y^2])
  2. jcsd
  3. May 21, 2006 #2
    So if A represents the area of the rectangle, you have [tex]\frac{dA}{dy}=4(\frac{-y^2}{\sqrt{4-y^2}}+\sqrt{4-y^2})[/tex].

    If you observe the expression, is there something you can factorize that will make it easier to solve for y when you set [tex]\frac{dA}{dy}=0[/tex]?
  4. May 21, 2006 #3
    Actually, from symmetry you can argue that the required rectangle has to be a square (special case of a rectangle) whose diagonals meet at the centre of the circle of radius 2 units .
    What can you say about the length of the side of this square ?
    Hint:Draw radii to the corners of the square .

    Of course if the symmetry isn't apparent, you can always go for the calculus approach, which involves setting up coordinate axes and maximising .
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