Quadratic equation. form four maths question

Let x cm be the length of each side of the original square card.
The side of the base of the box folded = (x - 8) cm
The height of the box folded = 4 cm
As the capacity = 400 cm3,
4(x - 8)2 = 400
(x - 8)2 = 100
x - 8 = 10　(x ＞ 0)
x = 18
So the length of each side of the original square card is 18 cm.

First, you must understand how the small squares are cut. I don't know how to explain to you in words. Therefore, if you understand, you can know that the box has a dimension of 4x^2 (Let the length of each side of the large square card be (4+x+4) cm)

Therefore, 4x^2=400
x^2=200
x^2-200=0
By quadratic formula, x= 14.1 or x= -14.1 (rejected)
x=14.1
Thus, the length of each side of the original square card is (4+14.1+4) = 22.1 cm

2007-10-20 16:57:29 補充：
when the four small squares are cut, the large square card will remain to be thick and cross-shaped. If you think from this direction, you can figure out that why the dimension of the box is 4x^2. Hope you think useful. Add oil! ^.^