google is your friend. See
https://www.teachoo.com/2194/584/Ex-3.3--22---Prove-cot-x-cot-2x---cot-2x-cot-3x---cot-3x-cot-x/category/Ex-3.3/
Prove:
Cot x cot 2x - cot 2x cot 3x - cot 3x cot x = 1
1 answer
Cot x cot 2x - cot 2x cot 3x - cot 3x cot x = 1
1 answer