$\displaystyle \frac{cos2x}{sin2x+1} = \frac{1-tanx}{1+tanx}$
Can't seem to prove this one, help please
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 $\displaystyle \sin (2x)$.