[IMG]file:///C:/DOCUME%7E1/Asus/LOCALS%7E1/Temp/moz-screenshot.jpg[/IMG]Cotx - tanx = 2cot2x
please help
You need to know that
and
.
These are proved using the sum formulas (which are proven here)
Proof of the sum and difference formulas.
Trigonometric identities
.