Solving this equation using the method of spherical means yields Kirchhoff’s formula in R3. It’s not the Schrödinger (10[1])equation, but applying the 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, as does using the curl-of-curl identity (10[1])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.” (10[1])This second-order PDE can be derived by treating a 1D string as a continuum of springs, yielding solutions that travel with speed “square-root of T over mu.” For 10 points, name this generically-named equation (10[1])that describes disturbances that propagate through media. ■END■ (10[5]0[2])