$\displaystyle \frac{cos2x}{sin2x+1} = \frac{1-tanx}{1+tanx}$

Can't seem to prove this one, help please

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- Nov 11th 2008, 06:24 PMRajProve Trig Identity
$\displaystyle \frac{cos2x}{sin2x+1} = \frac{1-tanx}{1+tanx}$

Can't seem to prove this one, help please - Nov 11th 2008, 06:58 PMmr fantastic
Start by choosing a side.

I choose the right hand side.

Substitute $\displaystyle \tan x = \frac{\sin x}{\cos x}$. Multiply numerator and denominator by $\displaystyle \cos x$.

Now multiply numerator and denominator by $\displaystyle \cos x + \sin x$ and expand.

You should recognise the numerator as $\displaystyle \cos (2x)$. Simplify the denominator to recognise $\displaystyle \sin (2x)$. - Nov 12th 2008, 07:30 AMRajQuote:

Substitute http://www.mathhelpforum.com/math-he...3a6cd29b-1.gif. Multiply numerator and denominator by http://www.mathhelpforum.com/math-he...3efbed9d-1.gif.

- Nov 12th 2008, 11:52 AMmr fantastic