# Exam 2010 Error in Question 2?

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 Exam 2010 Error in Question 2? October 17, 2011 01:13PM Registered: 10 years ago Posts: 114 Rating: 0
Hi Guys

I was going through the 2010 paper, and thought I might have come across a small error in the solution to question 2 b.

In the second row for the given Ry matrix, the cos(theta) in the second column should not be there, rather it should be a '1'. This seems to carry through as there should be a '1' instead of a '0' in the appropriate place in the solution that is provided for question b(i) and b(ii).

I was thinking that I was going insane, since I couldn't get the same answer as the solution answer, but if I am right, then this appears to be the error. Unless I've missed something else again...

EDIT:
There's also an error with the sign on the 'sin(theta)' entry in the first row of the same matrix: its negative when it should be positive.
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:03PM Registered: 10 years ago Posts: 72 Rating: 0
I was just busy with the same question.

Was gonna ask the same thing.

the bottom left sin should not have the minus, there should not be a cos in the middle.

ALSO!

If you think of the cartesian plane, 90 degrees clockwise, is -90 degrees... anticlockwise is +, that means, sin(-90) = -1

let me know what you think.
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:18PM Registered: 10 years ago Posts: 72 Rating: 0
o wait sorry,

the top sin should be a +sin
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:27PM Registered: 10 years ago Posts: 72 Rating: 0
I get my first rotation, Rxy takes the point "a" to :

-1
1
-1
1

and my other Ryx take the point "a" to

1
1
1
1

(unchanged) which makes sence when I imagine the situation

Rxy =
0 0 -1 0
1 0 0 0
0 -1 0 0
0 0 0 1

Ryx =
0 1 0 0
0 0 1 0
1 0 0 0
0 0 0 1

this always gets confusing, damn this matrix math, damn it to hell!
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:27PM Registered: 10 years ago Posts: 114 Rating: 0
Well, I'm trying to find some info on clockwise vs anti-clockwise.

To be honest, I'm not to concerned at the moment with which way it literally goes, as long as they say "45 degrees clockwise" or "60 degrees anti-clockwise", I should be fine... hopefully.

But yep... I actually missed that the question says that rotations are performed clockwise.. Which means that, as mentioned, the angle should be a value of '-90', not 90...

I'm wondering if the solution should be disregarded in this case, since it clearly doesn't fulfil the answer to the question...
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:29PM Registered: 10 years ago Posts: 72 Rating: 0
what did you get?

(I am ignoring the answer in the memo)
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:31PM Registered: 10 years ago Posts: 114 Rating: 0
Something else - having gone through the past papers posted on myUnisa... Is it just me, or do the questions generally get repeated? Maybe not in its entirety, but I've seen now 3 questions from different papers asking about homogenous coordinates. Not to mention the question about the textures...
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:40PM Registered: 10 years ago Posts: 72 Rating: 0
I also saw that.

Going through these old exams wil give us a VERY good idea of what to expect.

I also think the questions in self assesment B could be in the exam EXACTLY like that.

gonna tri my best to "parrot" those.
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:41PM Registered: 10 years ago Posts: 114 Rating: 0

Rxy:
0 0 1 0
1 0 0 0
0 1 0 0
0 0 0 1

Ryx:
0 1 0 0
0 0-1 0
-1 0 0 0
0 0 0 1

Then
a.Rxy = (1,1,1)
Seems a little wrong though...

a.Ryx = (1, -1, -1)
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:44PM Registered: 10 years ago Posts: 72 Rating: 0
I dont feel very convident in my answer to the previous matrix question,

this type of question is a given in the exam. I'll put money on it.
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:53PM Registered: 10 years ago Posts: 114 Rating: 0
I see my previous posted answers are based on my anti-clockwise rotation.

Rxy
0 0 -1 0
1 0 0 0
0 -1 0 0
0 0 0 0

Ryx
0 1 0 0
0 0 1 0
1 0 0 0
0 0 0 1

Thus:
a.Rxy = (-1, 1, -1)
a.Ryx = (1, 1, 1)

Thats with clockwise rotation...
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:53PM Registered: 10 years ago Posts: 72 Rating: 0
Thats looks exactly the way mine looked.

BUT

I forgot the -90... negative angle!

sin(90) = 1
sin(-90) = -1

I use this:

http://www.bluebit.gr/matrix-calculator/matrix_multiplication.aspx

when I'm in a hurry to check my answers.
 Re: Exam 2010 Error in Question 2? October 17, 2011 02:56PM Registered: 10 years ago Posts: 72 Rating: 0
Cool, that looks like mine now.

now I feel better,

we should be very carefull of that "clockwise, anticlockwise" vibe.