This means that you will have to resist the urge to use the formula whenever you encounter a frequency. The equations are too complicated to solve here, so we will use our mechanical analogy again. If the mass-spring system is driven at a frequency other than its natural frequency, a rather complicated motion results. The mass will start to oscillate, then stop, then start up again, and stop again, over and over. If we wait long enough, however, the motion at the natural frequency will die away because of friction, leaving only the motion at the driving frequency.
Electric circuits behave in the same way.
When you plug an appliance into the wall, an oscillating emf at 60 Hz is applied to it. For a very brief period of time there is a complicated variation of the current with time, but after this transient dies away, the current in the appliance oscillates steadily at 60 Hz.