00:01
So we'd like to express this matrix as a product of elementary matrices.
00:05
To do that i'm actually going to first just re -reduce it into the identity, it's not singular.
00:12
Step one, r2 is going to turn into r2 minus r1.
00:17
I'm going to subtract r1 from r2.
00:20
This becomes 1, 2, 0, minus 2.
00:25
Now have r1 become r1 plus r2.
00:30
1 plus 0 is 0, 2 plus negative 2 is 0.
00:35
And then i'm going to have r2 get divided by 2, or divided by negative 2.
00:42
So this becomes 1, 0, 0, 1.
00:46
Which is great, cool.
00:48
How is that useful to us? well if we do all these steps in reverse now, r2 times negative 2, r1 turns into r1 plus r2.
00:57
Do that in reverse, r1 turns into r1 minus r2.
01:07
And then r2 turns into r2 plus r1.
01:11
Just doing all these in reverse.
01:15
That's going to be, you can see, this matrix is 1, 0, 0, minus 2.
01:20
This is 1, minus 1, 0, 1.
01:24
And this is 1, that's all that.
01:30
Starting of course with the identity.
01:32
I'm going to claim that this product is this.
01:40
So we'll take, let's see, 1, 0, 0, 1.
01:48
Do this operation to it.
01:51
1, 0, minus 2, 0...