Wednesday, June 3, 2009


First off, don't mess this up:
Singular: matrix
Plural: matrices

Fun facts about matrices:)
A Matrix is a rectangular array of real numbers called entries (spreadsheet)

They have dimensions!
rows x columns (order matters)

Ex: Write dimensions of this matrix.

ANS: 2x3

You can add and subtract them!
But be careful kids... they must have the same dimensions


ANS: They're incompatible. Gotcha!


You can even multiply them together!
-AxB is possible if and only if the number of columns in A equals the number of rows in B

Ex: If A(2x3) and B(3x1), the answer will be (2x1)

Who are you matrix? Show us your identity

Matrix identities are defined, but only for square matrices (nxn)

for a 2x2:

for a 3x3:

ewww... what's that?

A DETERMINANT is any real number associated with any square matrix

-vertical bars on both sides of the matrix can also denote a matrix

Determinant of a 2x2 Matrix:

Coefficient Matrix Determinant

determinant (A) = IAI= ad-bc

NOTE: The determinant is the difference of the products of the 2 diagonals of the matrix

Ex: Find the determinant of the matrix

Ans: det(A)= 2(2)- 1(-3) = 4+3=7

Knowing this will make your life ten times easier, maybe even 100...
1. Hit 2nd matrix

2. Enter matrix as [A]

3. Arrow over to math and choose determinant feature(choice 1)

Determinant of a 3x3 matrix:
NOTE: its supposed to be a plus sign before a13 it wouldn't let me fix it!

Ex: Find IAI if



Now from this we can tell that determinants are so much fun...but what's even more fun is using them to find a 2X2 inverse!!

and det A= ad-bc

*switch negate

Guess what? The best part about matrices is... nothing! Haha just kidding! You can use them to solve systems of equations!


1) Reduced Row Echelon Form
2) Cramer's Rule
CRAMER'S RULE! because he's just that good!

Let's try an example to show how it works!

4x+y= 9

1st: Set up coefficient matrix

x y
IAI= 2-12= -10

2nd: Set up replacement matrices...we know its tedious but keep going, it's worth it in the end!

Replace x column: Replace y column:

3rd- Use Cramer's rule:

Isn't that cool!!!

Applications of Determinants!

You might be thinking where and when will I ever use this information in life?
Well, you will be suprised that there are many real life examples that you can amaze your friends with everyday!

1.)Have you ever randomly felt the urge to know the area of an object?

- well now you can easily find this answer on the spot with the matrix knowledge that you have just obtained!

Here's how:

All you need to do is plug the x values of the points into the first column, the y values into the second column and 1 into the third column of the matrix!

2.) You can also use this same method if three points are collinear if the area=0 and the equation of the line passing through a point!

Just look at all of the amazing things you can do with determinants!

Now that you are matrix masters yourselves, you should be anxious to try some problems on your own!!!

2.) Let A=Find A^2

3.) Find the inverse of: [4 -1]
[-2 1]

Solve for x and y:

8.) Solve using matrices:


9.) Find the area of the triangle with the vertices (-3,1), (3,0), and (5,4)

10.) Show, using determinants, that (-2,-5), (1,-2), and (5,2) are collinear.


1.)[9 -7]
[-9 4]
2.)[7 -18]
[9 34 ]

3.) [31 12]
[15 11 ]

4.)[2 -4 -6]
[-1 2 3]
[1 -2 -3]
5.) y=2

6.)x=6 x=-1
7.) 120, 240, and 0 degrees

8.)A=5 B=3 C=-2
9.) 13=area

10.) yes, collinear because area=0


Anonymous said...


kelseyyyy said...

your welcome!

Kate Nowak said...

Apparently there is not an easy way to format a matrix in LaTeX. :-(

I like your solution, but we need to figure out a way to line up the rows directly underneath each other.

Kate Nowak said...

I can't wait to see how this ends - I am in such suspense! I like the conversational tone. It makes it very easy to read.

Ellen + Kelsey said...

hey guys just wanted to say that i'm liking your blog and it's rly funny, but the white spaces are drving me insane. FYI :)

kelseyyyy said...

thank you! yea every time we add a matrix in it gets spaced out so it's really annoying but we're working on it!!

Kenneth Finnegan said...

You've got the sign reversed on the a13 term of the determinant. Each term alternates sign, so a11*det(sub matrix) - a12*det(sub matrix) + a13*det(sub matrix).
This also lets you calculate the determinant for larger matrices, but by then you may as well just plug it into your calculator.

julia said...

ok, i worked on this for over an hour in the library during 8th period and after school, and NONE of it saved... i only just noticed now, but i'm about to blow my matrix top off

Kate Nowak said...

Ack, I'm very sorry that happened. Not sure what to tell you. What do you still feel you need to do? The biggest thing I see is the error in the determinant formula. Don't get an ulcer over it. It's only math.

Kate Nowak said...

And btw, the coordinating profile pictures are a riot.

Anna said...

This was not only funny but I learned something too! Yayy

Nicole S. said...

ya guys this was helpful! you put a lot of info in it =)

Rachel E said...

this was a really helpful blog.
i never really got matrixs (i don't know the plural...), but this explained a lot.

kelseyyyy said...

haha MATRICES!! thats the first thing we said! but i'm glad you guys liked it and good luck tomorrow everybody!!!!!!!

julia said...

yeahhh boiiii we're blogging superstars! i hope we all get 100s on the final tomorrow. goooooooood luck!!