# mod arg form

Watch
Announcements
Thread starter 1 year ago
#1
why is it done this way? I know cos(x) = cos(-x) and -sin(x) = sin(-x)

and that z1/z2 = z1 - z2 = cos(x1 -x2) + isin(x1-x2) = cos(11x) = isin(11x) - the same answer as solution bank.

they've not even done: (z1/z2)(z*2/z*2)
Last edited by FurtherMaths2020; 1 year ago
0
1 year ago
#2
(Original post by FurtherMaths2020)
z1 - z2 = cos(x1 -x2) + isin(x1-x2)
This is not true. Where is this coming from?

Anyway, there is a typo on the second line. Obviously it should read

0
Thread starter 1 year ago
#3
(Original post by RDKGames)
This is not true. Where is this coming from?

Anyway, there is a typo on the second line. Obviously it should read

the textbook
0
Thread starter 1 year ago
#4
(Original post by RDKGames)

but if they did it like that, that following work wouldn't be the same. have a look at the answer again.
0
1 year ago
#5
(Original post by FurtherMaths2020)
but if they did it like that, that following work wouldn't be the same. have a look at the answer again.
RDKGames is right. Its a typo. You're multipying by the denom conjugate on the top and bottom. The bottom is unity magnitude (as its a unit vector). The top becomes
cos(11x) + isin(11x)
by addition formulae or more simply by de Moivre (if you've done it).
0
Thread starter 1 year ago
#6
(Original post by mqb2766)
RDKGames is right. Its a typo. You're multipying by the denom conjugate on the top and bottom. The bottom is unity magnitude (as its a unit vector). The top becomes
cos(11x) + isin(11x)
by addition formulae or more simply by de Moivre (if you've done it).
look at their answer - they've not multiplied through using the conjugate
0
1 year ago
#7
(Original post by FurtherMaths2020)
look at their answer - they've not multiplied through using the conjugate
They have, its a typo.
Line 3 is a scalar (magnitude - squared) on the denom. They've multiplied by the conjugate.

We are talking about Q17?
0
1 year ago
#8
(Original post by FurtherMaths2020)
look at their answer - they've not multiplied through using the conjugate
Yes they have!

cos(9x) + isin(9x) is the conjugate of cos(9x) - isin(9x)

0
1 year ago
#9
(Original post by FurtherMaths2020)
look at their answer - they've not multiplied through using the conjugate
It's a typo. Just multiply top and bottom by the complex conjugate of the bottom.

expand out and it works. where do you think cos^2+sin^2 has come from?
0
1 year ago
#10
(Original post by FurtherMaths2020)
the textbook
Yes what they show is the result for division, but you said z1 - z2 instead of z1/z2 therefore I said your claim is incorrect.
0
Thread starter 1 year ago
#11
(Original post by RDKGames)
Yes they have!

cos(9x) + isin(9x) is the conjugate of cos(9x) - isin(9x)

sorry, I see that they have. anyway, a per the textbook, I used the method they taught.
0
Thread starter 1 year ago
#12
(Original post by FurtherMaths2020)
why is it done this way? I know cos(x) = cos(-x) and -sin(x) = sin(-x)

and that z1/z2 = z1 - z2 = cos(x1 -x2) + isin(x1-x2) = cos(11x) = isin(11x) - the same answer as solution bank.

they've not even done: (z1/z2)(z*2/z*2)
(Original post by RDKGames)
Yes what they show is the result for division, but you said z1 - z2 instead of z1/z2 therefore I said your claim is incorrect.
I said z1/z2 = z1 - z2
0
Thread starter 1 year ago
#13
(Original post by mqb2766)
They have, its a typo.
Line 3 is a scalar (magnitude - squared) on the denom. They've multiplied by the conjugate.

We are talking about Q17?
ok, I see that. anyway, why haven't they done it this way: z1/z2 = z1 - z2 = cos(x1 -x2) + isin(x1-x2) = cos(11x) = isin(11x)
0
1 year ago
#14
These new spec. Edexcel textbooks have hundreds of mistakes in them across the 8 Maths and F Maths textbooks I have used (I do options FS1 and FM1). The teachers hate all the errors as well
0
Thread starter 1 year ago
#15
(Original post by NotNotBatman)
It's a typo. Just multiply top and bottom by the complex conjugate of the bottom.

expand out and it works. where do you think cos^2+sin^2 has come from?
ok, I see it now
0
1 year ago
#16
(Original post by FurtherMaths2020)
I said z1/z2 = z1 - z2
Yeah, hence automatically saying that division is the same as subtraction is incorrect.

Anyway, hopefully you understand how having z1 - z2 is incorrect in that line.
0
Thread starter 1 year ago
#17
(Original post by RDKGames)
Yeah, hence automatically saying that division is the same as subtraction is incorrect.

Anyway, hopefully you understand how having z1 - z2 is incorrect in that line.
yeah, I think I do. thanks
Last edited by FurtherMaths2020; 1 year ago
0
1 year ago
#18
(Original post by FurtherMaths2020)
yeah, I think I do
You think?

Well ok, let me put it this way;

It's like me saying

It's just a simple nonsensical error, nothing more to it. Just be careful in the future with what you write, that's the take-home message.
0
Thread starter 1 year ago
#19
(Original post by RDKGames)
You think?

Well ok, let me put it this way;

It's like me saying

It's just a simple nonsensical error, nothing more to it. Just be careful in the future with what you write, that's the take-home message.
sorry, I was confused with arg(z1/z2) = arg(z1) - arg(z2)
0
1 year ago
#20
(Original post by FurtherMaths2020)
sorry, I was confused with arg(z1/z2) = arg(z1) - arg(z2)
Because the question is based on arguments (imaginary exponentials, unity magnitude), this actually "works" on this question.
Last edited by mqb2766; 1 year ago
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### Do you think receiving Teacher Assessed Grades will impact your future?

I'm worried it will negatively impact me getting into university/college (20)
40%
I'm worried that I’m not academically prepared for the next stage in my educational journey (4)
8%
I'm worried it will impact my future career (2)
4%
I'm worried that my grades will be seen as ‘lesser’ because I didn’t take exams (13)
26%
I don’t think that receiving these grades will impact my future (6)
12%
I think that receiving these grades will affect me in another way (let us know in the discussion!) (5)
10%