#sec^2theta+1-3tantheta=0#
#=>1+tan^2theta+1-3tantheta=0#
#=>tan^2theta-3tantheta+2=0#
#=>tan^2theta-2tantheta-tantheta+2=0#
#=>(tantheta-2)tantheta-1(tantheta-2)=0#
#=>(tantheta-2)(tantheta-1)=0#
So
when #tantheta=2=tantan^-1(2)=tanalpha#
#=>theta=npi+alpha." where "n inZZ#
when #tantheta=1=tan(pi/4)#
#=>theta=npi+pi/4" where "n in ZZ#