To find the length of an arc expressed in terms of pi, you need to use the formula L = (n/2) * (C), where L is the length of the arc, n is the number of pi radians in the arc, and C is the circumference of the circle. If the circle has a radius of r, then the circumference is C = 2 * pi * r. Therefore, we can rewrite the formula as L = (n/2) * (2 * pi * r) = n * r * pi. This formula tells us that the length of an arc expressed in terms of pi is equal to the radius of the circle times the number of pi radians in the arc.