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Homework Help: Application on Differentiation

  1. Apr 17, 2009 #1
    1. The problem statement, all variables and given/known data
    could someone please help me to answer the following problem:

    Suppose a wire 20 cm long is to be cut into two pieces. One piece is to be bent in the shape of an equatorial triangle and the other in the shape of a circle. How should the wire be cut so as to:
    a) maximize the total area enclosed by the shapes ?

    b)minimize the total area enclosed by the shapes ?

    2. Relevant equations

    3. The attempt at a solution
    i applied x as the circle length and (20-x) as the triangle length ,,
    we know that x=(2pi)r > r=x/2pi ,, A(c)=pi*(x/2pi)^2 ,,
    A(c+t)= A(c) + A(t) ... then differentiate ,, solve for A`(c+t)=0 and i'll get what ?? maximize or minimize and how to get the other one ?? ,, and is there another way ?? (easier one) because it's hard to solve for A`(c+t) the equation is too long ...
  2. jcsd
  3. Apr 17, 2009 #2
    For the area of the triangle calculate the height by using the following:


    rather than using the Pythagorean Theorem. It will lead to a cleaner expression that will be much easier to differentiate. Take the second derivative of your expression to determine if it's a maximum or minimum.
  4. Apr 18, 2009 #3
    lol ,, thanks :)
  5. Apr 18, 2009 #4
    ok ,, i got A(c+t)= x^2/4pi + (sqrt(3)/36) * (20-x)^2
    i differentiate and solved for 0 i got x = 7.53583283 and got:
    the maximum A(20) and minimum A(x) ,, is it right ??
  6. Apr 20, 2009 #5
    If you have a graphing calculator, graph it and find the minimum. Otherwise, graph it on paper to see if your value is correct. Should be an easy check.
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