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By now, I'm sure you know
that in just about anything you do in life,
you need numbers.
In particular, though,
some fields don't just need a few numbers,
they need lots of them.
How do you keep track of all those numbers?
Well, mathematicians dating back
as early as ancient China
came up with a way to represent
arrays of many numbers at once.
Nowadays we call such an array a "matrix,"
and many of them hanging out together, "matrices".
Matrices are everywhere.
They are all around us,
even now in this very room.
Sorry, let's get back on track.
Matrices really are everywhere, though.
They are used in business,
and computer graphics.
One reason matrices are so cool
is that we can pack so much information into them
and then turn a huge series of different problems
into one single problem.
So, to use matrices, we need to learn how they work.
It turns out, you can treat matrices
just like regular numbers.
You can add them,
subtract them,
even multiply them.
You can't divide them,
but that's a rabbit hole of its own.
Adding matrices is pretty simple.
All you have to do is add the corresponding entries
in the order they come.
So the first entries get added together,
the second entries,
the third,
all the way down.
Of course, your matrices have to be the same size,
but that's pretty intuitive anyway.
You can also multiply the whole matrix
by a number, called a scalar.
Just multiply every entry by that number.
But wait, there's more!
You can actually multiply one matrix by another matrix.
It's not like adding them, though,
where you do it entry by entry.
It's more unique
and pretty cool once you get the hang of it.
Here's how it works.
Let's say you have two matrices.
Let's make them both two by two,
meaning two rows by two columns.
Write the first matrix to the left
and the second matrix goes next to it
and translated up a bit,
kind of like we are making a table.
The product we get when we multiply the matrices together
will go right between them.
We'll also draw some gridlines to help us along.
Now, look at the first row of the first matrix
and the first column of the second matrix.
See how there's two numbers in each?
Multiply the first number in the row
by the first number in the column:
1 times 2 is 2.
Now do the next ones:
3 times 3 is 9.
Now add them up:
2 plus 9 is 11.
Let's put that number in the top-left position
so that it matches up with the rows and columns
we used to get it.
See how that works?
You can do the same thing to get the other entries.
-4 plus 0 is -4.
4 plus -3 is 1.
-8 plus 0 is -8.
So, here's your answer.
Not all that bad, is it?
There's one catch, though.
Just like with addition,
your matrices have to be the right size.
Look at these two matrices.
2 times 8 is 16.
3 times 4 is 12.
3 times
wait a minute,
there are no more rows in the second matrix.
We ran out of room.
So, these matrices can't be multiplied.
The number of columns in the first matrix
has to be the same as the number of rows in the second matrix.
As long as you're careful
to match up your dimensions right, though,
it's pretty easy.
Understanding matrix multiplication
is just the beginning, by the way.
There's so much you can do with them.
For example, let's say you want
to encrypt a secret message.
Let's say it's "Math rules".
Though, why anybody would want to keep this a secret
is beyond me.
Letting numbers stand for letters,
you can put the numbers in a matrix
and then an encryption key in another.
Multiply them together
and you've got a new encoded matrix.
The only way to decode the new matrix
and read the message
is to have the key,
that second matrix.
There's even a branch of mathematics
that uses matrices constantly,
called Linear Algebra.
If you ever get a chance to study Linear Algebra,
do it, it's pretty awesome.
But just remember,
once you know how to use matrices,
you can do pretty much anything.


【TED-Ed】How to organize, add and multiply matrices - Bill Shillito

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Why Why 2013 年 3 月 28 日 に公開
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