Where do I start to prove this identity:

sinx/cosx= 1-cos2x/sin2x

please help!!

Hint: Fractions are evil. Get rid of them.

Well, cos2x = cos²x - sin²x, so
1-coscx = 1 - cos²x - sin²x =
1 - cos²x + sin²x
You should be able to simplify this to 2*something squared.
The denominator is sin2x = 2sin(x)cos(x)
You should be able to finish this, if not post a question.

The second line should be
1 - cos2x = 1 - (cos²x - sin²x)
then the 3rd line will make sense.
1 - cos²x should look familiar.