tan(cot-1x)=
Property: tan-1x+cot-1x=π2
Given that, tan(cot-1x)
So,
tan(cot-1x)=tanπ2–tan-1x [By using property]
=cot(tan-1x)
cot x cot 2x – cot 2x cot 3x – cot 3x cot x = 1