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 Post subject: cosinePosted: Fri, 2 Mar 2012 00:55:37 UTC

Joined: Mon, 27 Feb 2012 21:04:13 UTC
Posts: 5
Show that cos (3theta)=4cos^3(theta)-3cos(theta)

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 Post subject: Re: cosinePosted: Fri, 2 Mar 2012 02:11:31 UTC
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Joined: Wed, 30 Mar 2005 04:25:14 UTC
Posts: 12167
Location: Austin, TX
mirela wrote:
Show that cos (3theta)=4cos^3(theta)-3cos(theta)

.

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 Post subject: Re: cosinePosted: Fri, 2 Mar 2012 16:48:57 UTC
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Joined: Tue, 20 Oct 2009 18:18:45 UTC
Posts: 33
Location: Lisbon, Portugal
another way:

we know that:
1. cos(a+b) = cos(a).cos(b) - sen(a).sen(b)
2. cos(2theta) = cos^2(theta) - sin^2(theta)
3. cos(3theta)=cos(theta+2theta)

starting in 3., use 1., after this use 2. and use the relation sin^2(theta) = 1 - cos^2(theta)

Bye
Voxx

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 Post subject: Re: cosinePosted: Fri, 2 Mar 2012 17:42:58 UTC
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Joined: Wed, 30 Mar 2005 04:25:14 UTC
Posts: 12167
Location: Austin, TX
Voxx wrote:
another way:

we know that:
1. cos(a+b) = cos(a).cos(b) - sen(a).sen(b)
2. cos(2theta) = cos^2(theta) - sin^2(theta)
3. cos(3theta)=cos(theta+2theta)

starting in 3., use 1., after this use 2. and use the relation sin^2(theta) = 1 - cos^2(theta)

Bye
Voxx

If you advocate this route, you should really use

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