Math Question - How can I establish this identity?

[AgaBoogaBoo]

(cotO - tanO)/(cotO + tanO) = cos(2O)

O stands for theta

I just need a hint or something, thanks 🙂

 

[Goosemaster]

well, wihtout hurting my brain, remember that

tanx = sinx/cosx

cosx^2-sinx^2 = cos2x


That said, First, I would foil it out and then combine like terms etc


cot^2 + cottan - tancot -tan^2

cot^2 - tan^2

cos^2/sin^2 -sin^2/cos^2
i am sleepy as hell so i'll answer it tomorow

i think..

 

[AgaBoogaBoo]

I'll work with that and give it a try, thanks 🙂

 

[Goosemaster]

sorry i couldn't do more ..later..I'l waking up real early so I'll help you then

 

[AgaBoogaBoo]

That's fine, thanks anyway!

 

[vtqanh]

cotg(x) = cos(x) / sin(x)
tan(x) = sin(x) / cos(x)
so you will have [cos(x)/sin(x) - sin(x)/cos(x)] / [cos(x)/sin(x) + sin(x)/cos(x)]
That can easily be reduced to cos^2(x) - sin^2(x), which is cos(2x)
(you also need to remember that cos^2(x) + sin^2(x) = 1)
Good luck

 

[TuxDave]

Originally posted by: Goosemaster
well, wihtout hurting my brain, remember that

tanx = sinx/cosx

cosx^2-sinx^2 = cos2x


That said, First, I would foil it out and then combine like terms etc


cot^2 + cottan - tancot -tan^2

cot^2 - tan^2

cos^2/sin^2 -sin^2/cos^2
i am sleepy as hell so i'll answer it tomorow

i think..

I think you missed the divide symbol in the identity...