Question

the matrix -1,0 can be used in matrix multiplication
0,-1
for

A.
a rotation of 180º clockwise
B.
a reflection in the line y = -x
C.
a rotation of 270º counterclockwise
D.
a reflection in the y-axis

Answer 1

@ganeshie8

Answer 2

We just did this!!!

Answer 3

no we didnt

Answer 4

It's the exact same problem, in reverse.

Answer 5

i don't get if forward or reverse

Answer 6

Do you understand how the multiplication works?

Answer 7

si

Answer 8

a rotation of 180º clockwise

Answer 9

that means yes in spanish

Answer 10

Yo comprendo

Answer 11

muy bien

Answer 12

entonces es A

Answer 13

Do you understand the reflection (not rotation)?

Answer 14

yes

Answer 15

Not rotation.

Answer 16

yes

Answer 17

so its A

Answer 18

You know, I may have messed this up, gotta think for a second.. OK?

Answer 19

The problem before was reflection through the axis y = -x
That means that unit vector <1,0> should become <0,-1>
And unit vector <0,1> should become <-1,0> OK?

Answer 20

yes

Answer 21

And the matrix
0 -1
-1 0
when multiplied by either unit vector, does this

Answer 22

Therefore it will do it for any vector.

Answer 23

so its A

Answer 24

Now this matrix is not the one we had before, it is
-1 0
0 -1

Answer 25

yes

Answer 26

So my thinking is that it should reflect differently. I need to sketch it.

Answer 27

OK, so what does this vector give when multiplied by <1,0> and <0,1>

Answer 28

Can you do it?

Answer 29

it's A

Answer 30

Yes. The rotation matrix through an angle theta is
http://en.wikipedia.org/wiki/Rotation_matrix

cos theta - sin theta
sin theta cos theta

Answer 31

rotation through 180
sin 180 = sin 0 = 0
cos 180 =-1

Answer 32

Weird, huh? Moving the -1 off the diagonal and now we rotate.
Sorry it took me so long. Hasta luego