00:01
In this problem, we have a sphere like this and the radius is r.
00:09
And we are cutting this sphere by a plane, something like this.
00:16
And let's say the height of this piece is h.
00:22
Then the volume of this portion turns out to be, and the formula is given to be, okay, so this is the setup.
00:40
Now we want to cut this sphere with a plane so that the volume of this portion, let's say v1, is equal to 2 times the volume of this portion, let's say v2.
01:00
Okay, now we are told to assume that this distance is x.
01:08
So we want to find an equation that is satisfied by x according to this condition that says v1 equal to 2v2.
01:20
Okay, now that we know some part of the height, let us compute the entire height in terms of x.
01:28
So here is, okay we have r here, r plus x.
01:36
And this part, okay, noting that this is r, the smaller part is r minus x.
01:44
So now we know the height of each section, each portion, so we can write down their individual volumes.
01:53
V1 is equal to one third pi.
01:57
Okay, the height of v1 is r plus x minus 3r minus.
02:05
We have height again, r plus x.
02:12
Okay, before maybe v2, let us expand this expression.
02:16
If you carry out the multiplication and squaring, you will get 2 pi r cubed over 3 plus pi r squared x minus pi x cubed over 3.
02:34
Okay, now let us write down v2.
02:36
We have one third pi.
02:39
Height again, height is r minus x squared 3r minus r minus x.
02:49
And if you expand this, you will get 2 pi r cubed over 3 plus pi r squared x plus pi x cubed over 3.
03:05
So we want v1 equal to 2v2.
03:10
And we have this other requirement that says this is a unit sphere.
03:15
So r is equal to 1.
03:17
But i will do that at the end.
03:20
Okay, let's write v1.
03:22
Let's just copy it actually from here.
03:29
So this is v1...