Solving this equation using the method of spherical means yields Kirchhoff’s formula in R3. It’s not the Schrödinger equation, but applying the (10[1])WKB approximation to this equation yields the eikonal (“icon-al”) equation. Linearizing the metric in the Einstein field equations and transforming to the harmonic gauge gives an instance of this equation, (10[1])as does using the curl-of-curl identity on two of Maxwell’s equations. Any function of the form f of quantity x plus-or-minus vt solves this equation, which can be written as “the d’Alembertian (“dal-ahm-BARE-shin”) of u equals zero.” This second-order PDE (-5[1])can be derived by treating a 1D string as a continuum of springs, yielding solutions that travel with speed (10[1])“square-root of T over mu.” For 10 points, name this generically-named equation that describes disturbances that propagate through media. ■END■ (10[1]0[2])