The sine of 45 degrees is equal to the square root of 2 divided by 2. The sine function relates the ratio of the length of the side opposite to the angle to the length of the hypotenuse in a right triangle. In a 45-45-90 triangle, the two legs are congruent, and the hypotenuse is equal to the length of the leg multiplied by the square root of 2. Therefore, the length of the leg opposite the 45-degree angle is equal to the length of the hypotenuse divided by the square root of 2, which simplifies to the square root of 2 divided by 2.