My Math Forum Trig simplification

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 March 29th, 2010, 07:18 AM #1 Newbie   Joined: Mar 2010 Posts: 1 Thanks: 0 Trig simplification Hi guys, I've got a math issue regrding trig... I need to show that tan (4 arctan(1/5)) = 120/119 for a proof I'm doing. How would I go about doing/simplifying this? Thanks
 March 29th, 2010, 09:10 AM #2 Senior Member   Joined: Feb 2009 From: Adelaide, Australia Posts: 1,519 Thanks: 3 Re: Trig simplification Hi there; just use the identity $\tan 2x= \frac{2\tan x}{1-\tan^2 x}$ twice. That is, find tan 2x assuming you know that tan x = 1/5, then use that to find tan (2(2x)).

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