Why is sin90 equal to 1? 🥇 30.07.2019 Mathematics Answer: Let’s go to basics, what is sin(θ). Sin(θ) is the ration of opposite side to hypotenuse. In above figure B angle = 90°, now the opposite side is AC and even hypotenuse is AC. Sin(θ) = opposite/hypotenuse = AC/AC = 1.It’s easy!