C3 trig quick question

needsomeadvice

SOLVED
thanks :biggrin:

(edited 12 years ago)

Reply 1

F1's Finest

Original post by needsomeadvice

Express as a sum of cosines 5cos2xcos3x

I simplified this to 5/2(2cos2xcos3x) and got 5/2 (cos5x - cosx)

but in the textbook they simplified this to 5/2(2cos3xcos2x) and the answer is 5/2 (cos5x+cosx) :confused:

I don't understand why they changed the order of cos2xcos3x to cos3xcos2x and if this is allowed? :confused:

Is it okay to change the order like this e.g. change 5sin3xcosx to 5cosxsinx

Thnks in advance

Hi,

if you have the official C3 edexcel book ( thats if you are doing edexcel ) turn to page 124.

http://upload.wikimedia.org/wikipedia/en/math/7/6/4/7649e6e5a976127426eaff7b31f54b6f.png - check out this formula

i think now you should be able to do it :smile:

Reply 2

steve2005

Original post by needsomeadvice

I don't understand why they changed the order of cos2xcos3x to cos3xcos2x and if this is allowed? :confused:

Is it okay to change the order like this e.g. change 5sin3xcosx to 5cosxsinx

Thnks in advance

On the first line it should be plus rather than minus.

Uploaded with ImageShack.us

I guess you have made a typing error on your last line. The 3 has disappeared ??

(edited 12 years ago)

Reply 3

Akbar2k7

Original post by James A

Hey isnt that formula not part of our sylabus anymore thats what I have heard (Edexcel C3)

Reply 4

needsomeadvice

Original post by steve2005

On the first line it should be plus rather than minus.

Uploaded with ImageShack.us

I guess you have made a typing error on your last line. The 3 has disappeared ??

Yep i 4got the 3 :colondollar:

I get the formula its just they've changed the order of 2cos2xcos3x to 2cos3xcos2x, that's whats confusing me!

Reply 5

ElMoro

Original post by needsomeadvice

why is this simplified to 5/2(2cos3xcos2x) in the textbook
I get 5/2(2cos2xcos3x)

cos3x * cos2x = cos2x * cos3x

You're just timesing two things together. It doesn't matter what order you do it in :smile:

Reply 6

F1's Finest

Original post by Akbar2k7

Hey isnt that formula not part of our sylabus anymore thats what I have heard (Edexcel C3)

not too sure myself....

your doing pharmacy too !!

have you sent your ucas off yet?

Reply 7

needsomeadvice

Original post by ElMoro

cos3x * cos2x = cos2x * cos3x

You're just timesing two things together. It doesn't matter what order you do it in :smile:

But doesn't it make a difference if theres a number infront of one of them e.g. is 5cos3xcos2x and 5cos2xcos3x the same?

Reply 8

ElMoro

Original post by needsomeadvice

But doesn't it make a difference if theres a number infront of one of them e.g. is 5cos3xcos2x and 5cos2xcos3x the same?

There's no difference :nah:

5 * cos3x * cos2x = 5 * cos2x * cos3x = cos2x * 5 * cos3x = ...

When you times numbers together, the order doesn't matter. E.g.

2 * 3 = 3 * 2 = 6

4 * 3 * 2 = 2 * 4 * 3 = ... = 24

(edited 12 years ago)

Reply 9

Akbar2k7

Original post by James A

not too sure myself....

your doing pharmacy too !!

have you sent your ucas off yet?

Yeah on Friday.

Reply 10

F1's Finest

Original post by Akbar2k7

Yeah on Friday.

the wednesday coming up ill get it sent off by xD

im applying for nottingham and reading too !

Reply 11

Akbar2k7

Original post by James A

the wednesday coming up ill get it sent off by xD

im applying for nottingham and reading too !

They are great Universities I really hope my AS grades don't screw I just had a bad set of June exams, I am really capable of a lot more. I really want to go Nottingham since I would like go into research(maybe after a Phd) and Nottingham are like on of the best for research and have Russel Group status. Reading is just awesome for Mpharm.

Reply 12

F1's Finest

Original post by Akbar2k7

if you dont mind telling me, or pm if you want, what were your AS grades and in what subjects?

Reply 13

Akbar2k7

Original post by James A

if you dont mind telling me, or pm if you want, what were your AS grades and in what subjects?

In my sig bro ums an all im not too bothered cause I know I can and will do better in A2 and my resits

Reply 14

F1's Finest

Original post by Akbar2k7

In my sig bro ums an all im not too bothered cause I know I can and will do better in A2 and my resits

cool beans, was chemistry edexcel?? what about maths and bio?

my ums were

edexcel maths : 252/300 (A)
edexcel Chemistry 226/300 (B)
aqa biology 194/300 (C) lol biology is annoying.

nice ukcat score btw !!! you should definetly get A's in your resits !!

im resitting my unit 2 chem paper which was a badly set out paper by edexcel, retaking biology unit 1 because i got a D in it