00:01
In this question, we are given an open top box with square base.
00:06
The square base sites are length xcm and the height of the box is ycm.
00:12
We're given that the volume is fixed at 665 .5 cm cube.
00:16
We want to find the dimensions of the box where the minimum material will be used.
00:21
So first we need to form a formula for the surface area of the box.
00:26
I'll call it a.
00:29
And so it will be the area of the square base that will be x square plus area of the four sides now one side is area as xy and there are four of it so it will be four times xy now notice that there is two variable here so we need to substitute out the y now the hint will come from the volume now volume on the box is the square base x square times the height y and that will give us 665 .5 so my y is 665 .5 divided by x square.
01:04
So i'm going to sub that into the y over here.
01:17
So you end up with this plus two six six two over x.
01:24
I'm just going to bring it up to the top so x to power minus one.
01:29
Now let's differentiate a respect to x for the first term x square bring down the power to repeat the x subtract one to the power next term two six six two x to power one.
01:41
Leave the constant alone.
01:45
Bring down the power minus one, repeat the x and subtract one from the power.
01:53
So you will get this...