Question: Which of the following is true for all angles θ?

Options:

1. sin(θ) = cos(90° - θ)
2. tan(θ) = sin(θ) + cos(θ)
3. sec(θ) = 1/tan(θ)
4. csc(θ) = sin(θ)

Correct Answer: sin(θ) = cos(90° - θ)

Solution:

sin(θ) = cos(90° - θ) is a co-function identity.