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An open box is to be made from a flat piece of material 18 inches long and 2 inches wide by cutting equal squares of length xfrom the corners and folding up the sides.
Write the volume Vof the box as a function of x. Leave it as a product of factors, do not multiply out the factors.

If we write the domain of the box as an open interval in the form (a,b), then what is a=?
and what is b=?

I got V=x(18-2x)(2-2x)
and a=0

but it says that its not correct

  • math -

    Your equation V=x(18-2x)(2-2x) , which is correct, describes the volume for any given x

    clearly V has to be a positive number.
    notice that for x=9 and x=1, V is zero.

    for all values of x between 1 and 9 , V is positive,
    for x<1 and x > 9 , V is negative.
    try a number between 1 and 9

    Draw your conclusion from my hints.

  • math -

    the equation you got is correct,, =)
    first, note that x (which is the height of the box) cannot be negative, since there is no negative height,,
    equating V to zero,
    x=0, 1, 9

    now, get some values in between (and if you want, get also a value greater than 9), and check if the dimensions will all be positive, and V will be positive as well,,
    if x = 0.5, V = (0.5)(17)(1) = 8.5
    if x = 4, V = (4)(10)(-6) = -240
    if x = 10, V = (10)(-2)(-18) = 360

    note that even if the volume that we got for x = 10 is positive, the length and width are negative, which cannot happen,,
    therefore, domain is (0,1)

    so there,, i hope i was able to help.. =)

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