Which of the following statements is true?

The cofunction identity is sin θ = cos(90° − θ). Thus sin 18° = cos 72°, so statement a is true, and sin 72° = cos 18°, so statement c is true. Statement b is not true because sin 55° is not equal to cos 55°. Therefore, the correct choice is “Both a and c.”

AI-written and checked against the verified answer.