The question follows like this:

Show that the formula for the T.S.A, A (in square meters) of the box is A=-6x^2 + 12x

The information that was given is:

8x + 4y=12

also

y= 3 - 2x

Printable View

- Apr 7th 2011, 05:38 AMRosieTotal Surface Area problem.
The question follows like this:

Show that the formula for the T.S.A, A (in square meters) of the box is A=-6x^2 + 12x

The information that was given is:

8x + 4y=12

also

y= 3 - 2x - Apr 7th 2011, 05:54 AMHallsofIvy
What you have written makes no sense at all. You ask for the total surface area of a box but say nothing about how "x" and "y" are related to the box. Perhaps x and y are two of the dimensions (length and height, say) but what is the third dimension (width)? Also, you say, "8x+ 4y= 12 also y= 3- 2x" but those are, in fact, equations of same line in the xy-plane.

I would like to help but I really have no idea what the problem really**is**. - Apr 7th 2011, 06:20 AMRosie
Sorry, y is the lenght and x is breadth, the was bo imformation given on the hight. It is a rectangular frame.

- Apr 7th 2011, 07:00 AMHallsofIvy
If the dimensions of the rectangular solid are x, y, and z, then the total surface area is 2xy+ 2yz+ 2xz. If y= 3- 2x, then the total surface area is $\displaystyle 2x(3- 2x)+ 2(3- 2x)z+ 2xz= 6x- 4x^2+ 6z- 4xz+ 2xz= 6z- 4xz+ 6x- 4x^2$. In order for that to be equal to $\displaystyle 12x- 6x^2$ we must have $\displaystyle 6z- 4xz+ 5x- 4x^2= 12x- 6x^2$ which reduces to $\displaystyle (6- 4x)z= 7x- 2x^2$ or $\displaystyle z= \frac{7x- 2x^2}{6- 4x}$. Was there any condition like that in your problem?

- Apr 7th 2011, 07:06 AMRosie
NO. Here is the sum in full is.

A piece of wire 12m long, is bent to form a rectangular frame. The lenght is y m and the breadth and the height are x m. We can deduce that: 8x + 4y = 12

If you make y the subject it becomes y= 3 - 2x

The whole frame is coverd with cardboard to make a box.

Show that the formuls for the T.S.A, A (in square metres) of the box is A= -6x^2 + 12x. - Apr 7th 2011, 07:45 AMHallsofIvy
"The breadth

**and**the height are x m". Yes, you**are**given the z component! It is the same as the x component. The total surface area is, as I said before, 2xy+ 2yz+ 2xy and, because we now know that the problem said that x= z. 2xy+ 2yz+ 2xy= 2xy+ 2xy+ 2x^2= 4xy+ 2x^2. Now, put y= 3- 2x into that. - Apr 7th 2011, 08:01 AMRosie
Thank you very much.

- May 22nd 2011, 11:19 PMrithika
Surface Area of a rectangular geometry would be given as Area = 2(wh + lw + lh). To get the easier conversions on area or surface unit you may check out with the

Area Unit Converter, comprising of imperial conversion units.