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HSC 2015 MX2 Marathon (archive) (7 Viewers)

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InteGrand

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Re: HSC 2015 4U Marathon

Whats the working out?


What this is equivalent to is saying that is a negative real number. Let .

.

As the imaginary part 2xy must be zero, we have or .

If , then , which is true for all real y, hence all points on the y-axis lie on the locus (as these are the points where x = 0 and y is any real number).

If , then , so all points on the y-axis strictly between x = -2 and x = +2 are included.
 

braintic

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Re: HSC 2015 4U Marathon

I didn't mention a region. I referred to the INTERVAL between (-2,0) and (2,0).

There is no need to combine the arguments. It is MUCH easier to visualize rather than doing algebra.

It is simply a case of playing with vectors. Try experimenting with different placements of z, and examine the values of arg(z-2) and arg(z+2) - graphically that is, not algebraicly - and seeing when they add to pi.
 
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InteGrand

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Re: HSC 2015 4U Marathon

I didn't mention a region. I referred to the INTERVAL between (-2,0) and (2,0).

There is no need to combine the arguments. It is MUCH easier to visualize rather than doing algebra.

It is simply a case of playing with vectors. Try experimenting with different placements of z, and examine the values of arg(z-2) and arg(z+2) - graphically that is, not algebraicly - and seeing when they add to pi.
It's pretty easy to visualise if one knows or has seen the answer already, but I think it can be tricky to come up with the answer if one has never seen this type of question before based on just playing with vectors.
 

braintic

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Re: HSC 2015 4U Marathon

It's pretty easy to visualise if one knows or has seen the answer already, but I think it can be tricky to come up with the answer if one has never seen this type of question before based on just playing with vectors.
I guess it depends on how visual a thinker someone is. For me, I find visual solutions much easier (and much more satisfying) than pure algebraic solutions. And I wouldn't be prepared to label someone a 'maths wiz' until they can visualize these kinds of things.
 
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Kaido

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Re: HSC 2015 4U Marathon

^ just draw the line between -2 and 2
 

InteGrand

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Re: HSC 2015 4U Marathon

How'd you get that inequality ?
I understood all of that except that inequality?
is a negative real number, since its arg is , which means its real part is negative (since negative real numbers have real part < 0), and its imaginary part is 0 (since the imaginary part of any real number is 0).
 

Drsoccerball

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Re: HSC 2015 4U Marathon

is a negative real number, since its arg is , which means its real part is negative (since negative real numbers have real part < 0), and its imaginary part is 0 (since the imaginary part of any real number is 0).
*Facepalm*
 

Kaido

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Re: HSC 2015 4U Marathon

i did that and ekman said that it was the wrong way to do it o_O maybe i screwed up
In hsc, they dont expect you to algebraically prove this locus type q.
this is one of those geometrical locus :D
 
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