Pre-Calc

An open box with a volume of 1500cm^3 is to be constructed by taking a piece of cardboard of 20 cm by 40 cm, cutting squares of sides length x cm from each corner and folding up the sides. Show that this can be done in two different ways, and find exact dimensions of the box in each case.

I have already found the solution using the quadratic equation, can someone help me find the solution using synthetic division?

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asked by Andy G
  1. L = 40 - 2x
    w = 20 -2x

    V = 1500 = L w x
    1500= (40-2x)(20-2x)x = (800 -120 x + 4 x^2)x
    so
    4x^3 - 120 x^2 + 800 x -1500 = 0
    make a reasonable guess
    x = 5 for example
    then the box would be 30*10 * 5 = 1500
    good, height of five is a solution
    so divide by (x-5)

    x^3 - 120 x^2 + 800 x -1500
    -----------------------------
    (x-5)

    = 4 x^2-100 x +300

    factor that
    4 (x^2- 25 x + 75) = 0
    I get about 21 which is too big to take off the width and 2.89
    check x = 2.89
    20 - 5.78 = 14.22
    40 - 5.78 = 34.22
    v = 14.22*34.22*2.89 = 1406
    well, better do it a bit more accurately

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    posted by Damon

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