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## #1 2008-11-22 22:23:49

tony123
Member
Registered: 2007-08-03
Posts: 189

### cos(x - y)

Prove that if

and
,
then

Last edited by tony123 (2008-11-22 22:30:46)

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## #2 2008-11-23 11:23:59

JaneFairfax
Member
Registered: 2007-02-23
Posts: 6,868

Easy!

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## #3 2008-11-23 11:27:04

Daniel123
Member
Registered: 2007-05-23
Posts: 663

### Re: cos(x - y)

By inspection,

Last edited by Daniel123 (2008-11-23 11:27:22)

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## #4 2008-11-23 11:49:28

JaneFairfax
Member
Registered: 2007-02-23
Posts: 6,868

### Re: cos(x - y)

You need to do a bit more work than that.

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## #5 2008-11-24 09:16:32

greenisfab88
Member
Registered: 2008-11-24
Posts: 3

### Re: cos(x - y)

I have troublr with that too.
i need help!

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## #6 2008-11-24 13:50:06

JaneFairfax
Member
Registered: 2007-02-23
Posts: 6,868

### Re: cos(x - y)

By the way, you also need the condition

to ensure that
and
are not both
.

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## #7 2010-01-22 15:58:56

jonecox
Member
Registered: 2010-01-20
Posts: 3

### Re: cos(x - y)

I know very well this expression.I am studying this function in school.I like your work.Thanks for share this information.

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## #8 2010-09-06 01:17:29

sameer mishra
Member
Registered: 2010-08-27
Posts: 64

### Re: cos(x - y)

cosx +cosy=a
sinx-siny=b
squarring bot the equation then subtrac
cos2x+cos2y+2cox(x-y)=a2-b2
2cos(x-y)[1+cos(x+y)]=a^2-b^2  (o)

now squarring above equation then add
cos(x+y)=(a^2+b^2)/2-1
now put the valu of cos(x+y) in (o)
cos(x-y)=a2 -b2/a2+b2

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## #9 2010-09-16 22:54:26

sameer mishra
Member
Registered: 2010-08-27
Posts: 64

### Re: cos(x - y)

hello JaneFairfax i think that you are intresested in trigo.
but i challange you in trigo. calculus algebra. coordinate pleaseeeeeeeee
accept my challenge and ask question from me

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