`2 tan theta- cot theta=-1`
or `2 tan theta- 1/(tan theta)=-1`
or `2 tan^(2) theta+tan theta-1=0`
or `(tan theta+1) (2 tan theta-1)=0`
or `tan theta=-1 or tan theta=1/2`
or `tan theta=tan ((-pi)/4) or tan theta=tan ("tan"^(-1) 1/2)`
`rArr theta=n pi +((-pi)/4)` or `theta=mpi+alpha`,
where `m, n in Z` and `tan alpha =1/2`.
