It is given that

Then, we have

…………………… (1)

…………………… (2)

Therefore, from equation (1) and (2), we get

, [since, cos2t= (2cos²t-1) and also cos2t= (1-2sin²t)]

, [Since, cos3t = 4cos³t-3cost, sin3t = 3sint – 4sin³t]

= -cot3t

Hence, the value of is -cot3t