The sine of the angle is the ratio of the opposite side to the hypotenuse. The hypotenuse of this triangle can be computed from the Pythagorean theorem: .. AB = √(AC^2 +BC^2) = √(9 +4) = √13 . . . . cm Then the ratio of opposite side BC to the hypotenuse AB is .. sin(A) = BC/AB = 2/√13