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# How do i prove this? watch

1. tan(3x)=3tanx-tan^3(x)/1-3tan^2x
2. (Original post by jeja12)
tan(3x)=3tanx-tan^3(x)/1-3tan^2x
Split into tan(2x + x).

That gives you (tan 2x + tan x) / (1 - tan 2x tan x)

now use tan 2x = 2 tan x / (1- tan^2 x).
3. thx
4. (Original post by jeja12)
thx
No problem.
5. Also how can I express cos4a in terms of cosA only.
6. (Original post by Zacken)
cos 4a = cos (2(2a)) = 2cos^2 2a - 1= 2(1-cos^2 a)^2 - 1
how did you factorise 2cos^2 2a -1 into that
7. (Original post by jeja12)
how did you factorise 2cos^2 2a -1 into that
What I wrote was wrong, I'll delete it, sorry.

What I meant was: cos 4a = cos (2(2a)) = 2cos^2 2a - 1

The cos 2a = 2cos^2 a - 1

So plug that in:

2 (cos 2a)^2 -1 becomes 2(2cos^2 a -1)^2 - 1
8. I get it now thank you very much
9. (Original post by jeja12)
I get it now thank you very much
No problem!

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Updated: June 13, 2016
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