Question

How cot[1800-@]=-cot@

Answer 1

$\cot(180 - \alpha ) = \frac{ \cos(180 - \alpha ) }{ \sin(180 - \alpha )}$

and

$\cos(180 - \alpha ) = - \cos( \alpha )$

$\sin(180 - \alpha ) = \sin( \alpha )$

so,

$\cot(180 - \alpha ) = \frac{ - \cos( \alpha ) }{ \sin( \alpha ) }$

therefore,

$\cot(180 - \alpha ) = - \cot( \alpha )$

hope this helps