help again (1 Viewer)

[vizman]

rofl. sorry bout another thread. can't seem to get this question

prove that cos ( A + B ) cos ( A - B ) = cos^2a - sin^2b


thnx.

 

[mitsui]

ah shit. i haf no idea
i onli got as far as

cos(A+B)cos(A-B)
= (cosAcosB-sinAsinB)(cosAcosB+sinAsinB)
= con^2Acos^2B-sin^2Asin^2B

..awaits the rest

 

[Templar]

= cos^2Acos^2B-sin^2Asin^2B

=cos^2A(1-sin^2B)-sin^2B(1-cos^2A)
=cos^2A-cos^2Asin^2B-sin^2B+cos^2Asin^2B
=cos^2A-sin^2B

 

[mitsui]

oh yea. xD thx

 

[vizman]

thanks so much guys

 

[SoulSearcher]

isnt this extension 1 work?

 

[insert-username]

isnt this extension 1 work?

Yes, trig equations are Extension 1 and not 2U work. 2U just cover the Pythagorean trig identities.


I_F