A pendulum 18in. in length swings 72 degrees from left to right. what is the horizontal distance between where the pendulum begins and ends?

We can use the Law of Cosines to solve this.....the horizontal distance, D, is given by:

D   =  sqrt  [ 18^2 + 18^2  - 2(18)(18)cos(72) ]  ≈ 21.16 inches