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Edexcel A2 C4 Mathematics June 2015 - Official Thread

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Original post by Pineappleface317
Hey :smile: I'm actually OCR but I couldn't find an OCR C4 2015 thread but I'm guessing you guys do vectors??
Could someone help me with this question:

Find a unit vector that is perpendicular to both 4i + 10j - 3k and 5i - 9j + 7k

All I have so far is:
4a + 10b - 3c = 0
and
5a - 9b + 7c = 0

(ai + bj + ck being my unknown unit vector)

Thanks :smile:


Hint: it says "unit" vector

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Original post by Pineappleface317
Hey :smile: I'm actually OCR but I couldn't find an OCR C4 2015 thread but I'm guessing you guys do vectors??
Could someone help me with this question:

Find a unit vector that is perpendicular to both 4i + 10j - 3k and 5i - 9j + 7k

All I have so far is:
4a + 10b - 3c = 0
and
5a - 9b + 7c = 0

(ai + bj + ck being my unknown unit vector)

Thanks :smile:


Maybe use c=1 and solve


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Original post by aersh8
Hint: it says "unit" vector

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A very poor hint - if it could be considered a hint at all.
Original post by Medicjug
Maybe use c=1 and solve


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Probably better to use c=0

But c=1 would also give a perpendicular vector - just easier when using c=0
Original post by Medicjug
Maybe use c=1 and solve


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just cross them!!!!!! :frown:
Original post by Medicjug
Maybe use c=1 and solve


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So that's just because each component in i k and k is one unit long? So I could make anything one?
Sorry for some reason I can't get my head around vectors ..
Original post by Tow
Probably better to use c=0

But c=1 would also give a perpendicular vector - just easier when using c=0


How come you can use c=0 ?
Original post by Pineappleface317
So that's just because each component in i k and k is one unit long? So I could make anything one?
Sorry for some reason I can't get my head around vectors ..


No, think of it this way if som vector is parralel to both it will be a mutiple of some direction vector. We can factor out a multiple such that c=1 leaving a,b not determined as of yet and then adjust the actual vector in the end.


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Original post by Tow
Probably better to use c=0

But c=1 would also give a perpendicular vector - just easier when using c=0


You cant use Z=0 this results in the vector 0,0,0.


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Original post by physicsmaths
You cant use Z=0 this results in the vector 0,0,0.


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Oh yeah.

I assumed it would be easier - haven't done this question since we went through the book back in Feb!

But yeah you use c=1 sorry about that xD

It can be anything BUT 0 and obviously 1 would be the easiest choice.
Original post by Tow
A very poor hint - if it could be considered a hint at all.


To be fair that can be used to form 3 simultaneous equations but that'll just be harder to solve

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Original post by Tow
Oh yeah.

I assumed it would be easier - haven't done this question since we went through the book back in Feb!

But yeah you use c=1 sorry about that xD

It can be anything BUT 0 and obviously 1 would be the easiest choice.


Correct.


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Reply 692
Avatar for TeeEm
TeeEm
19
useful material for the upcoming exams

http://www.thestudentroom.co.uk/showthread.php?t=3353445

http://www.thestudentroom.co.uk/showthread.php?t=3361867

http://www.thestudentroom.co.uk/showthread.php?t=3361905
Reply 693
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lam12
2
Original post by Tow
Yeah definitely.

But no question should require you to use the 'Loop' method.

Integration by parts like this is fine:
x^2sin(x)

But integration by parts of this shouldn't come up:
Sin(3x)cos(x)

Unless you can rearrange to make it so that you can integrate it with using the loop method.


I don't know the integration by loops method, what should I use to work this out?


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Original post by lam12
I don't know the integration by loops method, what should I use to work this out?


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Work what out?
Reply 695
Avatar for lam12
lam12
2
Original post by Tow
Work what out?


The integration of the sin2x whatever it was :-)


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Original post by lam12
The integration of the sin2x whatever it was :-)


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I think a previous post said that you can rearrange to integrate so try that.

I can't do it right now so if you still don't understand I'll do it when I'm free.
do we need to know the integration of tanx cotx secx and cosecs? :s-smilie:
Original post by crepole
do we need to know the integration of tanx cotx secx and cosecs? :s-smilie:


They are in the formula book. You could be asked to differentiate one of the ln ones to get to tanx or something along those lines.


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Does anyone know why you need a minus sign infront of the constant when setting up a diff. equation and its decreasing? thanks

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