y=cot x =sinx/cosx= 1/tanx

Step 1: y' = 2cotx cot' x,

Step 2, y'' = 2cot' x cot' x+2cot x cot'' x,

Step 3, find cot'x and cot''x and substitute it into Y''.

If you are not familiar with COT 'x =-CSC 2 (x) and CSC 'x =-CSCX COTX. Cot x =sinx/cosx can be selected for derivation, which is not complicated.