The simplest exact solution of free vibration of a string with timevarying length is presented. Taking into account virtual sources of waves which can be assigned to reflecting waves, an extended Stokes' formula is obtained. Coordinates of the virtual sources of the waves which can be observed at (t, x) are given by (0, -at+x) and (0, at+x). Using values of the virtual sources, the solution at (t, x) can be obtained.
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