The Edexcel C4 (21/06/12 - PM) Revision Thread

Original post by soempty

What is the easiest way to determine that angle between two vectors is acute? Is it just that dot product will be negative?

I'd just use the scalar product to find the cosine of the angle, find the actual angle, and check if it was above 270 or below 90 degrees. Either of these means there's an acute angle between the two.

Reply 783

Original post by Brand New Eyes

I agree although you're looking at 94UMS roughly depending on boundaries but I see! and you could always retake C4 in January? D:

I just want an A* for pride purposes :getmecoat:

Ah! So Maths/Economics/Physics?

Economics. Top unis aren't too fond of retakes, I believe, and that would mean the A* in Maths would arrive too late for me to apply with it, making it useless and probably shutting me out of Oxbridge/UCL/LSE completely. :frown:

Reply 784

Original post by soempty

What is the easiest way to determine that angle between two vectors is acute? Is it just that dot product will be negative?

Well, scalar product uses a cosine function. If the dot product is positive then the angle is acute. If the dot product is negative then the angle is obtuse...

Reply 785

kozo

3

Original post by soempty

What is the easiest way to determine that angle between two vectors is acute? Is it just that dot product will be negative?

There is always an acute angle between two vectors if their is an obtuse angle.
Taking the modulus of a.b will ensure you get the acute angle but as stated above, just think of where cos is +/- to establish whether its the acute or obtuse.

Reply 786

Original post by Llewellyn

Well, scalar product uses a cosine function. If the dot product is positive then the angle is acute. If the dot product is negative then the angle is obtuse...

I forgot that.
I would have added in the extra step of finding the actual angle rather than using the cosine value...

(edited 11 years ago)

Reply 787

Original post by kozo

There is always an acute angle between two vectors if their is an obtuse angle.

Sorry, but that's wrong.

Reply 788

kozo

3

Original post by Arva

Sorry, but that's wrong.

You are saying if two direction vectors intersect there will, in some cases, only be an acute OR obtuse angle....?

Reply 789

grazie

2

Original post by Jorgeyy

Whoever negged you for that is despicable. +rep

Edit: Oh, it won't even let me rep you because it's been too frequent, I think I have a crush on your maths ability 8-)

Cheers

Does that mean you'll swap your C3 exam paper for mine? Didn't go as well as would have liked. :frown:

(edited 11 years ago)

Reply 790

Original post by Puloot

guys, why differentiation of U= 2^x is U'= 2^x . ln2 ??? HELPP PLEASE!

Remember that

f(x)

is equivalent to

e^{\ln(f(x))}

Using that identity would be the simplest way to differentiate here.

Extension if you want more: differentiate x^x

Reply 791

If anyone wants any help I'd be happy to give my input. I did C4 this Thursday so I've just come off quite a bit of revision.

Reply 792

Original post by Llewellyn

Remember that

f(x)

is equivalent to

e^{\ln(f(x))}

Using that identity would be the simplest way to differentiate here.

Extension if you want more: differentiate x^x

I like to do it implicitly instead :dontknow:

.

Reply 793

TheNoobyPotato

Original post by Arva

I'd just use the scalar product to find the cosine of the angle, find the actual angle, and check if it was above 270 or below 90 degrees. Either of these means there's an acute angle between the two.

Above 270? Aren't acute angles less than 90? :s-smilie:

Reply 794

Original post by kozo

You are saying if two direction vectors intersect there will, in some cases, only be an acute OR obtuse angle....?

obtuse = less than 180. Acute = less than 90. The two can never add up to 360 degrees, ever.

Reply 795

Original post by TheNoobyPotato

Above 270? Aren't acute angles less than 90? :s-smilie:

Yep, but the two angles between a vector will always add up to 360 degrees - so if the result you get is more than 270, you can tell that the other angle is less than 90 and thus acute. :smile:

Reply 796

Original post by Arva

I like to do it implicitly instead :dontknow:

.

You would have to take a logarithm first if you wanted to differentiate implicitly, but that is essentially the same method. People have different preferences and that's fine so long as you get the correct result.

Reply 797

kozo

3

Original post by Arva

obtuse = less than 180. Acute = less than 90. The two can never add up to 360 degrees, ever.

I never once stated that. I have stated if you have an obtuse angle between two intersecting direction vectors then you also have an acute angle.
That has nothing to do with them adding to give 360 degrees.

Reply 798

Brand New Eyes

2

Original post by Arva

Economics. Top unis aren't too fond of retakes, I believe, and that would mean the A* in Maths would arrive too late for me to apply with it, making it useless and probably shutting me out of Oxbridge/UCL/LSE completely. :frown:

Would probably matter most for LSE, but very true, and Warwick. As long as you're predicted A* you can put the overall grade as "pending" on UCAS and declare a unit to be taken at a later date. It could just be that your school takes C4 in Jan (we do that at mine for the FM group)

Oxford only asks for A :smile:

but judging by your aptitude I'm sure you will! Good luck.

I'm not sure which paper to do...

For anyone who's curious to grade boundaries over the years:

Picture 12.png

Reply 799