# mod arg form

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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)

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

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#2

(Original post by

z1 - z2 = cos(x1 -x2) + isin(x1-x2)

**FurtherMaths2020**)z1 - z2 = cos(x1 -x2) + isin(x1-x2)

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

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(Original post by

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

**RDKGames**)**This is not true. Where is this coming from?**Anyway, there is a typo on the second line. Obviously it should read

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#5

(Original post by

but if they did it like that, that following work wouldn't be the same. have a look at the answer again.

**FurtherMaths2020**)but if they did it like that, that following work wouldn't be the same. have a look at the answer again.

cos(11x) + isin(11x)

by addition formulae or more simply by de Moivre (if you've done it).

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(Original post by

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).

**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).

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#7

(Original post by

look at their answer - they've not multiplied through using the conjugate

**FurtherMaths2020**)look at their answer - they've not multiplied through using the conjugate

Line 3 is a scalar (magnitude - squared) on the denom. They've multiplied by the conjugate.

We are talking about Q17?

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#8

**FurtherMaths2020**)

look at their answer - they've not multiplied through using the conjugate

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

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#9

**FurtherMaths2020**)

look at their answer - they've not multiplied through using the conjugate

expand out and it works. where do you think cos^2+sin^2 has come from?

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#10

(Original post by

the textbook

**FurtherMaths2020**)the textbook

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(Original post by

Yes they have!

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

**RDKGames**)Yes they have!

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

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(Original post by

why is it done this way? I know cos(x) = cos(-x) and -sin(x) = sin(-x)

and that

they've not even done: (z1/z2)(z*2/z*2)

**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

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.

**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.

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(Original post by

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?

**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?

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#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

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(Original post by

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?

**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?

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#16

(Original post by

I said z1/z2 = z1 - z2

**FurtherMaths2020**)I said z1/z2 = z1 - z2

Anyway, hopefully you understand how having z1 - z2 is incorrect in that line.

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(Original post by

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.

**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.

Last edited by FurtherMaths2020; 1 year ago

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#18

(Original post by

yeah, I think I do

**FurtherMaths2020**)yeah, I think I do

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.

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(Original post by

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.

**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.

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#20

(Original post by

sorry, I was confused with arg(z1/z2) = arg(z1) - arg(z2)

**FurtherMaths2020**)sorry, I was confused with arg(z1/z2) = arg(z1) - arg(z2)

Last edited by mqb2766; 1 year ago

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