16. Now it is your turn!
      Make Standing Wave

Start another column at F18 for the sum of the two waves. Then add this
 wave to the chart. You would have then created the standing waves!
This is left as an exercise for you to do on your own.

REMEMBER that when you add the third wave (the sum of yr and yl)
the scale of y-axis get switched back to auto. DON'T forget to disable it
by un-clicking the auto for both maximum and minimum.