Use these two identities:
cottheta=1/tantheta=1/(sintheta/costheta)=costheta/sinthetacotθ=1tanθ=1sinθcosθ=cosθsinθ
sectheta=1/costhetasecθ=1cosθ
Here's the expression:
color(white)=cotthetasecthetasintheta=cotθsecθsinθ
=cottheta*sectheta*sintheta=cotθ⋅secθ⋅sinθ
=costheta/sintheta*1/costheta*sintheta=cosθsinθ⋅1cosθ⋅sinθ
=color(red)cancelcolor(black)costheta/sintheta*1/color(red)cancelcolor(black)costheta*sintheta
=1/sintheta*1*sintheta
=1/sintheta*sintheta
=1/color(red)cancelcolor(black)sintheta*color(red)cancelcolor(black)sintheta
=1
That's it. Hope this helped!