1. The problem statement, all variables and given/known data I'm reading through the derivations of the linear wave equation. I'm following everything, except the passage I highlighted in yellow in the below attachment: 2. Relevant equations I'm not understanding why partials must be used because "we evaluate this tangent at a particular instant of time." 3. The attempt at a solution It seems if x, y, and t were all changing then there'd be a need for a partial. As stated in the highlighted text, I'm not getting it. Any help is appreciated.