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Modeling with Equations

  1. Sep 25, 2008 #1
    1. The problem statement, all variables and given/known data
    I am supposed to be tutoring pre-calc, and they are currently studying modeling with equations, i haven't have pre-calculus in years, and there are no sufficient examples in this book and i could use assistance helping these kids! I feel like I should know how because the problems seem fairly simple. problem example:

    Find two numbers whose sum is -24 and whose product is a maximum.

    I now the answer is -12,-12, but am not sure how to show the work.

    2. Relevant equations

    A rectangle is inscribed in a semicircle of radius 10. find a function that models the area A of the rectangle in terms of its height h.

    3. The attempt at a solution
  2. jcsd
  3. Sep 25, 2008 #2


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    Think of each clause as an equation. As soon as you see "two numbers" you should think "let x be one of the numbers and y the other" (or "a and b" or "u and v", ...). In fact, I would recommend being as specific as possible: "let x be the larger of the two numbers and y the smaller". Now, "two numbers whose sum is -24" means x+ y= what? "whose product is a maximum" means you want to find the maximum value of xy.

    Draw a picture. Obviously, you draw a semicircle of radius 10 with a rectangle inside it.("Inscribed" means the vertices of the rectangle all lie on the semi-circle, either on the circle itself or on the base line).
    Because you are given a geometric description but want an equation, I would recommend setting up a coordinate system. Choose your coordinates so the origin is at the center of the circle, the base of the semicircle is the x-axis, and the semicircle lies above the x- axis.
    What is the equation of the semicircle in that coordinate system?

    Actually this could be a very complicated problem but because of the reference to "the area A in terms of its height", you are clearly expected to assume one side of the rectangle (the "width") is along the base line of the semicircle. That simplifies things a lot!

    Since the base of the rectangle lies on the x-axis, and the height is measured perpendicular to that, the height is the y value. A vertex above that is on the circle. Taking y= h, use the equation of the circle to solve for x. There will be two solutions because there are two such points. Now, from that, what is the width of the rectangle in terms of h? Area= height*length for a rectangle.

    3. The attempt at a solution[/QUOTE]
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