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  #1  
Old 05-07-2013, 01:33 PM
Ziad Hatahet Ziad Hatahet is offline
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Default *ANSWER* Question 2

Plugging in \tilde{w_1} > 0, \tilde{w_2} < 0 gives the following plot: http://www.wolframalpha.com/input/?i=x%5E2-y%5E2+%3D+1

Yet, it seems that choice [e] was incorrect according to the answer key. But choice [d] (as per the answer key) gives the following plot: http://www.wolframalpha.com/input/?i...%2By%5E2+%3D+1

What am I missing?
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  #2  
Old 05-07-2013, 02:01 PM
Michael Reach Michael Reach is offline
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Default Re: *ANSWER* Question 2

I think you are adding the constant term on the wrong side of the equation.
w0*1 + w1*x^2 + w2*y^2
To get these to add up to +1 in the middle of the plot (x=0, y=0, say), we'll need w0 = -1.
Then to get negative values as x increases ( --> -1 on the right side of of the plot), we'll need w1 to be negative. And so on.
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Old 05-07-2013, 02:06 PM
dlammerts dlammerts is offline
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Default Re: *ANSWER* Question 2

Ziad,
I made the same mistake and finally figured out that the shape of the hyperbolas in your two plots is determined by the fact that your w0 is negative (just rearrange your formulas into the h = sign(w0 + w1*x1^2 + w2*x2^2) form to see this). The "inner" part of your hyperbolas assumes the value -1 instead of +1 as required. If your plot answer [d] with a positive w0, which makes the inner region assume the value +1, then the the orientation of the hyperbolas flips to the required shape:
http://www.wolframalpha.com/input/?i...Bx2%5E2+%3D-+1

Dirk
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Old 05-07-2013, 02:07 PM
Elroch Elroch is offline
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Default Re: *ANSWER* Question 2

What happens if you shunt everything to the same side of the equation?

[EDIT: snap, Michael ]
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  #5  
Old 05-07-2013, 03:40 PM
Ziad Hatahet Ziad Hatahet is offline
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Default Re: *ANSWER* Question 2

Thank you everyone for the replies. It is somewhat annoying to make a mistake on an "easy" question
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Old 05-07-2013, 08:35 PM
Michael Reach Michael Reach is offline
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Default Re: *ANSWER* Question 2

Join the club! Throughout the course, I have struggled on the programming problems and generally get them right eventually - and consistently get half of the "easy" ones wrong on some dumb mistake or another.
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