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Maximize area

  1. Feb 3, 2004 #1
    Here is the question

    You are planning to close off a corner of the first quadrant with a line segment 20units long running from (a,0) to (0,b). Show that the area of the triangle enclosed by the segment is largest when a = b.
  2. jcsd
  3. Feb 4, 2004 #2

    And also Apply the Distance formula to find the relation b/w a and b eliminate one from the two equations and use calculus to find the max of AREA
  4. Feb 4, 2004 #3


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    Staff Emeritus
    Science Advisor

    Or use "Lagrange multiplier" method:

    To maximize (1/2)ab subject to the requirement that x/a+ y/b= 1 (the equation of the line from (a,0) to (0,b)) we must have
    The vector (1/2)b i+ (1/2)a j (the grad of (1/2)ab) parallel to the vector (1/a)i+ (1/b)j (the grad of x/a+ y/b) (in an "ab" coordinate system of course).
    That is (1/2)b= λ(1/a) and (1/2)a= λ(1/b) where λ is the Lagrange multiplier. Dividing the first equation by the second to eliminate λ, b/a= a/b or a2= b2 so a= b or a= -b. Since this is in the first quadrant, a= b.
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