please answer, i dont understand how to go about this problem.

First note:

y=sec

^{-1}(1/t) ⇔ 1/t = sec y = 1/cosy ⇔ t = cos yNow take the t-derivative of t = cos y:

1 = -(sin y) y' = -y' √(1-cos²y) = -y' √(1-t²)

so that

y' =-1/√(1-t²)