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slim2010 · May 10, 2008, 09:24 AM

2 cot 2x=cot x-tan x

galactus · May 10, 2008, 09:35 AM

Often with problems like these it is helpful to convert everything to sin and cos.

Then you get from the identity $tan(x)=\frac{sin(x)}{cos(x)}$

$\frac{2cos(2x)}{sin(2x)}=\frac{cos(x)}{sin(x)}-\frac{sin(x)}{cos(x)}$

Now, $cos(2x)=cos^{2}(x)-sin^{2}(x), \;\ sin(2x)=2sin(x)cos(x)$ :

So, we have:

$\frac{2(cos^{2}(x)-sin^{2}(x))}{2sin(x)cos(x)}=\frac{cos^{2}(x)-sin^{2}(x)}{sin(x)cos(x)}$

Which equals the right side.