00:01
So in this problem, we're given three vectors in r3, and they all have their initial points at the origin.
00:07
We're asked if these all three lie in the same plane.
00:11
So for that to happen, what we can determine is if v1 as a dot product with v2 cross v3 is 0.
00:25
If it's zero, then they lie in the same plane.
00:28
Okay, so first of all, let's do v2 cross v3.
00:39
So we build our little matrix here, i, j, and k, and we put the v2 here, 2, 4, negative 6, and 360.
00:53
We do the determinant of that.
00:57
So for the co -factor for i, we have a little 2 by 2 here of 4 -9 660, so the down diagonal is 0 minus the up diagonal, which is negative 36.
01:11
So minus minus is plus.
01:14
And then we have minus j times its 2 by 2 matrix.
01:20
2, negative 6, 3, 0.
01:21
So 2 times 0 is 0, minus 3 times negative 6 is negative 18.
01:28
So negative negative is plus 18, plus k times the terminant of its 2 by 2 matrix.
01:37
Three, six.
01:38
So two times six is 12 minus three times four is 12.
01:46
Okay.
01:47
So what do we end up here? we end up with 36i minus 18j plus zero k, all right? okay.
02:04
Now v1 dot product with v2 cross v3 gives us.
02:14
Negative 1, 2, 3, dot product with 36, negative 18, 0.
02:27
Remember dot product, you multiply each of the elements and then add them all together, right? so you go minus 1 times 36, that's minus 36.
02:39
And then 2 times minus 18, that's minus 36, and then 3 times 0 is 0.
02:53
And so that's negative 72, which is not zero.
03:11
Okay, so i just realized i made a mistake right here.
03:14
This is supposed to be negative.
03:16
Right, v2 is supposed to be 2, negative 4, negative 6, and v3 is 360.
03:21
So i made a little error copying over.
03:25
All right, so what does that do for me? that doesn't change i.
03:29
It doesn't change j.
03:32
It changes k because this is now negative 12.
03:36
3 times negative 4 is negative 12.
03:39
And so then this is no longer zero right here.
03:45
Okay, so let's fix this up.
03:50
So this is negative 24k.
03:58
So that's negative 24 there.
04:02
So then three times negative 24, that's all right? three times negative 24.
04:29
Wait a minute.
04:31
Negative 12 minus 12 was a plus.
04:51
Let's see for k, this is i'll get this right here just a second.
04:58
Bear with me.
04:58
Here we go.
05:00
Let's look at the two by two matrix for k.
05:02
2, negative 4, 36.
05:04
So 2 times 6 is 12.
05:09
And then minus, 3 times negative 4 be negative 12.
05:15
Minus minus is plus 12...