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Exam 2010 Error in Question 2?

Posted by wishblade 
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Exam 2010 Error in Question 2?
October 17, 2011 01:13PM
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.
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:03PM
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.
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:18PM
o wait sorry,

the top sin should be a +sin
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:27PM
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
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...
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:29PM
what did you get?

(I am ignoring the answer in the memo)
Re: Exam 2010 Error in Question 2?
October 17, 2011 02:31PM
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...
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:40PM
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
Hmmm, well here's my answers:

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)
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:44PM
I dont feel very convident in my answer to the previous matrix question,

Please do it like you would do it, and post your answers if you dont mind.

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
I see my previous posted answers are based on my anti-clockwise rotation.

Here's my clockwise rotation answers:

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...
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:53PM
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.
avatar Re: Exam 2010 Error in Question 2?
October 17, 2011 02:56PM
Cool, that looks like mine now.

now I feel better,

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

mental note made.

Thanks bro, we'll speak soon.
Re: Exam 2010 Error in Question 2?
October 17, 2011 02:58PM
I guess your answers and mine seem to add up for the clockwise rotation... smoking smiley

I would just be a little worried about the parrot learning - I tried that last year in one of my exams... I left the studying a little too late, and ended up trying to learn just the solutions to a past paper or two. Luckily I didn't end up writing that day (work commitments), but if I had (having seen the paper afterwards), I think it would have gone a little south as I would have turned the first page in the exam hall...
Re: Exam 2010 Error in Question 2?
October 17, 2011 02:59PM
Just glad I'm through the one hurdle...Now for the rest of them...
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